<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e41e0-c180-79b0-9a83-ba25ac545b6d,g:92126a41-5611-4ed7-b2be-9b46fb2bc9a8-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYUnaudited" xlink:type="simple" xlink:href="tndm-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYUnaudited"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e41e0-c3b4-74ae-a480-bf9504099fa5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e41e0-c3b4-7708-9144-9ecae2a392a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e41e0-c3b4-74ae-a480-bf9504099fa5" xlink:to="loc_us-gaap_StatementTable_019e41e0-c3b4-7708-9144-9ecae2a392a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e41e0-c3b4-7cb4-8355-39b42a90c53f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e41e0-c3b4-7708-9144-9ecae2a392a5" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e41e0-c3b4-7cb4-8355-39b42a90c53f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-7cb4-8355-39b42a90c53f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e41e0-c3b4-7cb4-8355-39b42a90c53f" xlink:to="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-7cb4-8355-39b42a90c53f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-77ec-a0ab-91f31a676918" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e41e0-c3b4-7cb4-8355-39b42a90c53f" xlink:to="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-77ec-a0ab-91f31a676918" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e41e0-c3b4-7bb6-85a2-17f72b21952e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-77ec-a0ab-91f31a676918" xlink:to="loc_us-gaap_CommonStockMember_019e41e0-c3b4-7bb6-85a2-17f72b21952e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e41e0-c3b4-760b-915f-8a64c637308e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-77ec-a0ab-91f31a676918" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e41e0-c3b4-760b-915f-8a64c637308e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e41e0-c3b4-77ac-8de3-c888ce78e561" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-77ec-a0ab-91f31a676918" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e41e0-c3b4-77ac-8de3-c888ce78e561" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e41e0-c3b4-70e4-8fa1-7bb3b09b31f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e41e0-c3b4-77ec-a0ab-91f31a676918" xlink:to="loc_us-gaap_RetainedEarningsMember_019e41e0-c3b4-70e4-8fa1-7bb3b09b31f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e41e0-c3b4-74ae-a480-bf9504099fa5" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e41e0-c3b4-7f71-94ba-b58dd42f943a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_SharesOutstanding_019e41e0-c3b4-7f71-94ba-b58dd42f943a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e41e0-c3b4-7e69-bf0b-cf74d2365bff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_StockholdersEquity_019e41e0-c3b4-7e69-bf0b-cf74d2365bff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e41e0-c3b4-7baa-a648-eb56dae66b52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e41e0-c3b4-7baa-a648-eb56dae66b52" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e41e0-c3b4-7263-bafe-bc53857aa79b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e41e0-c3b4-7263-bafe-bc53857aa79b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_019e41e0-c3b4-747f-918b-963ca3d18a55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_019e41e0-c3b4-747f-918b-963ca3d18a55" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_019e41e0-c3b4-7b5c-94b8-02a70149fc22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_019e41e0-c3b4-7b5c-94b8-02a70149fc22" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f0296-efcc-7cd8-bb49-1ee7db6bfff2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f0296-efcc-7cd8-bb49-1ee7db6bfff2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019f0296-efcc-7396-9f59-47854aea64fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019f0296-efcc-7396-9f59-47854aea64fa" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e41e0-c3b4-79bd-af10-a493c09e24cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e41e0-c3b4-79bd-af10-a493c09e24cb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebtPurchaseOfCappedCallOption_019e41e0-c3b4-729f-9134-c1ee0687699d" xlink:href="tndm-20260630.xsd#tndm_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebtPurchaseOfCappedCallOption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_tndm_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebtPurchaseOfCappedCallOption_019e41e0-c3b4-729f-9134-c1ee0687699d" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_019e41e0-c3b4-7122-b306-57b0c8f98af6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_019e41e0-c3b4-7122-b306-57b0c8f98af6" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent_019e41e0-c3b4-761a-ac84-c211666a9816" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent_019e41e0-c3b4-761a-ac84-c211666a9816" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_019e41e0-c3b4-7f3b-86c4-e05c17fc6c0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_019e41e0-c3b4-7f3b-86c4-e05c17fc6c0a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e41e0-c3b4-79e2-9f66-08fc5a56944d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e41e0-c3b4-7e73-b086-19b79346f361" xlink:to="loc_us-gaap_NetIncomeLoss_019e41e0-c3b4-79e2-9f66-08fc5a56944d" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e41e0-c3b4-7b97-9958-d5f9e5b87865" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e41e0-c3b4-7f87-ac0e-2db8a7617bf3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/SummaryofSignificantAccountingPoliciesAdditionalInformationDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#SummaryofSignificantAccountingPoliciesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/SummaryofSignificantAccountingPoliciesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:href="tndm-20260630.xsd#tndm_SummaryOfSignificantAccountingPoliciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_SummaryOfSignificantAccountingPoliciesTable_019e41e0-c3b4-7347-8741-8ebe832920ea" xlink:href="tndm-20260630.xsd#tndm_SummaryOfSignificantAccountingPoliciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_tndm_SummaryOfSignificantAccountingPoliciesTable_019e41e0-c3b4-7347-8741-8ebe832920ea" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f02ba-efd3-7778-b321-8aa312ad466a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesTable_019e41e0-c3b4-7347-8741-8ebe832920ea" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f02ba-efd3-7778-b321-8aa312ad466a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f02ba-efd3-7778-b321-8aa312ad466a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f02ba-efd3-7778-b321-8aa312ad466a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f02ba-efd3-7778-b321-8aa312ad466a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f02ba-efd3-7284-997d-1b65e41fcd37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f02ba-efd3-7778-b321-8aa312ad466a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f02ba-efd3-7284-997d-1b65e41fcd37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PatentsMember_019f02ba-efd3-7580-8cac-9c8054076543" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PatentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f02ba-efd3-7284-997d-1b65e41fcd37" xlink:to="loc_us-gaap_PatentsMember_019f02ba-efd3-7580-8cac-9c8054076543" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f02cc-91c1-7312-b241-832562754b2e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesTable_019e41e0-c3b4-7347-8741-8ebe832920ea" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f02cc-91c1-7312-b241-832562754b2e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f02cc-91c1-7312-b241-832562754b2e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f02cc-91c1-7312-b241-832562754b2e" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f02cc-91c1-7312-b241-832562754b2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f02cc-91c1-74d2-b5d2-8294ff6ad71b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f02cc-91c1-7312-b241-832562754b2e" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f02cc-91c1-74d2-b5d2-8294ff6ad71b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_CeQurSAMember_019f02cc-91c1-7d8e-bb02-ec1344af3cfe" xlink:href="tndm-20260630.xsd#tndm_CeQurSAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019f02cc-91c1-74d2-b5d2-8294ff6ad71b" xlink:to="loc_tndm_CeQurSAMember_019f02cc-91c1-7d8e-bb02-ec1344af3cfe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f02d1-547f-7cb4-90a1-b326371d23cd" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesTable_019e41e0-c3b4-7347-8741-8ebe832920ea" xlink:to="loc_srt_ProductOrServiceAxis_019f02d1-547f-7cb4-90a1-b326371d23cd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f02d1-547f-7cb4-90a1-b326371d23cd_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f02d1-547f-7cb4-90a1-b326371d23cd" xlink:to="loc_srt_ProductsAndServicesDomain_019f02d1-547f-7cb4-90a1-b326371d23cd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f02d1-547f-791b-b27e-261e68865ea8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f02d1-547f-7cb4-90a1-b326371d23cd" xlink:to="loc_srt_ProductsAndServicesDomain_019f02d1-547f-791b-b27e-261e68865ea8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_InsulinPumpMember_019f02d1-547f-7c22-81d5-f16b94eced9b" xlink:href="tndm-20260630.xsd#tndm_InsulinPumpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f02d1-547f-791b-b27e-261e68865ea8" xlink:to="loc_tndm_InsulinPumpMember_019f02d1-547f-7c22-81d5-f16b94eced9b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_SingleUseInsulinCartridgesAndInfusionSetsMember_019f02d1-8ef3-7446-88d1-eced9f1e200c" xlink:href="tndm-20260630.xsd#tndm_SingleUseInsulinCartridgesAndInfusionSetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f02d1-547f-791b-b27e-261e68865ea8" xlink:to="loc_tndm_SingleUseInsulinCartridgesAndInfusionSetsMember_019f02d1-8ef3-7446-88d1-eced9f1e200c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019f02ba-efd3-76f4-9e40-3f3bae4e14c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LitigationSettlementAmountAwardedToOtherParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019f02ba-efd3-76f4-9e40-3f3bae4e14c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_LitigationSettlementPaymentTerm_019f02ba-efd3-76e0-9b7a-9ffd4f6a3bc3" xlink:href="tndm-20260630.xsd#tndm_LitigationSettlementPaymentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_tndm_LitigationSettlementPaymentTerm_019f02ba-efd3-76e0-9b7a-9ffd4f6a3bc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForLegalSettlements_019f02ba-efd3-7a75-8f70-1ad29557c33f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForLegalSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_PaymentsForLegalSettlements_019f02ba-efd3-7a75-8f70-1ad29557c33f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_LitigationSettlementNumberOfInstallmentTerms_019f02ba-efd3-75b3-a8ea-0d053d5aa8ee" xlink:href="tndm-20260630.xsd#tndm_LitigationSettlementNumberOfInstallmentTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_tndm_LitigationSettlementNumberOfInstallmentTerms_019f02ba-efd3-75b3-a8ea-0d053d5aa8ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019f02ba-efd3-7cab-a503-926e93fdfce7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019f02ba-efd3-7cab-a503-926e93fdfce7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019f02ba-efd3-7379-b120-b760e287f8c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019f02ba-efd3-7379-b120-b760e287f8c4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetImpairmentCharges_019f02bb-ee87-73a2-af02-d3ed76b3ae63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_AssetImpairmentCharges_019f02bb-ee87-73a2-af02-d3ed76b3ae63" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019f02cf-696b-75db-9e33-52afd2f29586" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_EquityMethodInvestments_019f02cf-696b-75db-9e33-52afd2f29586" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_019f02cc-91c1-7f61-af8f-2e454fcb16e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_019f02cc-91c1-7f61-af8f-2e454fcb16e2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019f02cc-91c1-72e1-b5bf-f236cf03b36d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019f02cc-91c1-72e1-b5bf-f236cf03b36d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment_019f02da-a584-7d14-bfdc-2f79f2a569cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment_019f02da-a584-7d14-bfdc-2f79f2a569cc" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019f02d0-ef76-71ef-aa5d-0f99bc6c93c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019f02d0-ef76-71ef-aa5d-0f99bc6c93c2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ProductsWarrantyPeriod_019f02d1-547f-7723-b2d0-2b14da092229" xlink:href="tndm-20260630.xsd#tndm_ProductsWarrantyPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_SummaryOfSignificantAccountingPoliciesLineItems_019f02ba-efd3-7283-8bd7-862567cdc872" xlink:to="loc_tndm_ProductsWarrantyPeriod_019f02d1-547f-7723-b2d0-2b14da092229" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/SummaryofSignificantAccountingPoliciesScheduleofAntiDilutiveSecuritiesDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#SummaryofSignificantAccountingPoliciesScheduleofAntiDilutiveSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/SummaryofSignificantAccountingPoliciesScheduleofAntiDilutiveSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e41e0-c3b4-724b-9c01-de300014ed78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e41e0-c3b4-781c-b252-6df7f306a561" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e41e0-c3b4-724b-9c01-de300014ed78" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e41e0-c3b4-781c-b252-6df7f306a561" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c3b4-75fe-aad0-39c63cfaf785" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e41e0-c3b4-781c-b252-6df7f306a561" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c3b4-75fe-aad0-39c63cfaf785" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-75fe-aad0-39c63cfaf785_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c3b4-75fe-aad0-39c63cfaf785" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-75fe-aad0-39c63cfaf785_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-70b5-8dd5-cac8c90c1edf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c3b4-75fe-aad0-39c63cfaf785" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-70b5-8dd5-cac8c90c1edf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e41e0-c3b4-7a99-833c-12c2bd7f7b3f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-70b5-8dd5-cac8c90c1edf" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e41e0-c3b4-7a99-833c-12c2bd7f7b3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e41e0-c3b4-70e7-a894-6407e5121b2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-70b5-8dd5-cac8c90c1edf" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e41e0-c3b4-70e7-a894-6407e5121b2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_019e41e0-c3b4-7a56-917e-2b1f45fd76ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-70b5-8dd5-cac8c90c1edf" xlink:to="loc_us-gaap_WarrantMember_019e41e0-c3b4-7a56-917e-2b1f45fd76ca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e41e0-c3b4-7fd5-8a00-05885058c16d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-70b5-8dd5-cac8c90c1edf" xlink:to="loc_us-gaap_EmployeeStockMember_019e41e0-c3b4-7fd5-8a00-05885058c16d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_019e41e0-c3b4-7f78-ab11-9e8cd5a051d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c3b4-70b5-8dd5-cac8c90c1edf" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_019e41e0-c3b4-7f78-ab11-9e8cd5a051d2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e41e0-c3b4-79ec-9412-26dbfd64ea02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e41e0-c3b4-724b-9c01-de300014ed78" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e41e0-c3b4-79ec-9412-26dbfd64ea02" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/ShortTermInvestmentsScheduleofEstimatedFairValueofShortTermInvestmentsDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#ShortTermInvestmentsScheduleofEstimatedFairValueofShortTermInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/ShortTermInvestmentsScheduleofEstimatedFairValueofShortTermInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-7e19-b3a6-d0be93e1a093" xlink:href="tndm-20260630.xsd#tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsTable_019e41e0-c3b4-7cd5-9380-b78b4c11c758" xlink:href="tndm-20260630.xsd#tndm_CashCashEquivalentsAndShortTermInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-7e19-b3a6-d0be93e1a093" xlink:to="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsTable_019e41e0-c3b4-7cd5-9380-b78b4c11c758" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7001-984e-7751f95413fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsTable_019e41e0-c3b4-7cd5-9380-b78b4c11c758" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7001-984e-7751f95413fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7001-984e-7751f95413fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7001-984e-7751f95413fd" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7001-984e-7751f95413fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-745d-b9e8-2c419bc7aea0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7001-984e-7751f95413fd" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-745d-b9e8-2c419bc7aea0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember_019e41e0-c3b4-7067-a1b3-3d278908a1dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-745d-b9e8-2c419bc7aea0" xlink:to="loc_us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember_019e41e0-c3b4-7067-a1b3-3d278908a1dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e41e0-c3b4-7507-a38c-047ca584091b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-745d-b9e8-2c419bc7aea0" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e41e0-c3b4-7507-a38c-047ca584091b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e41e0-c3b4-7980-837d-2fe4fe32fcd1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-745d-b9e8-2c419bc7aea0" xlink:to="loc_us-gaap_CommercialPaperMember_019e41e0-c3b4-7980-837d-2fe4fe32fcd1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e41e0-c3b4-7655-b15c-6f86b2d986cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-745d-b9e8-2c419bc7aea0" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e41e0-c3b4-7655-b15c-6f86b2d986cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract_019e41e0-c3b4-7294-997c-b39243140177" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-7e19-b3a6-d0be93e1a093" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract_019e41e0-c3b4-7294-997c-b39243140177" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019e41e0-c3b4-7f20-a7ff-53c01a2ad040" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract_019e41e0-c3b4-7294-997c-b39243140177" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019e41e0-c3b4-7f20-a7ff-53c01a2ad040" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e41e0-c3b4-79b6-9d92-51caf7c6e7f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract_019e41e0-c3b4-7294-997c-b39243140177" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e41e0-c3b4-79b6-9d92-51caf7c6e7f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e41e0-c3b4-7bea-8dd5-21012fe1a537" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract_019e41e0-c3b4-7294-997c-b39243140177" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e41e0-c3b4-7bea-8dd5-21012fe1a537" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e41e0-c3b4-7f53-be7e-b06d4d81491c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract_019e41e0-c3b4-7294-997c-b39243140177" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e41e0-c3b4-7f53-be7e-b06d4d81491c" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/ShortTermInvestmentsScheduleofContractualMaturitiesofAvailableForSaleDebtSecuritiesDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#ShortTermInvestmentsScheduleofContractualMaturitiesofAvailableForSaleDebtSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/ShortTermInvestmentsScheduleofContractualMaturitiesofAvailableForSaleDebtSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-73b5-b570-49021dbd30f8" xlink:href="tndm-20260630.xsd#tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsTable_019e41e0-c3b4-7118-9a4d-51e48188a63f" xlink:href="tndm-20260630.xsd#tndm_CashCashEquivalentsAndShortTermInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-73b5-b570-49021dbd30f8" xlink:to="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsTable_019e41e0-c3b4-7118-9a4d-51e48188a63f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7e7d-8d76-a1ef5852c748" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsTable_019e41e0-c3b4-7118-9a4d-51e48188a63f" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7e7d-8d76-a1ef5852c748" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7e7d-8d76-a1ef5852c748_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7e7d-8d76-a1ef5852c748" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7e7d-8d76-a1ef5852c748_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7947-9bfe-2a484e3baf0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b4-7e7d-8d76-a1ef5852c748" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7947-9bfe-2a484e3baf0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember_019e41e0-c3b4-7fc9-936f-2c2b9db5fdb5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7947-9bfe-2a484e3baf0b" xlink:to="loc_us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember_019e41e0-c3b4-7fc9-936f-2c2b9db5fdb5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e41e0-c3b4-7088-ab27-287cc2a2b190" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7947-9bfe-2a484e3baf0b" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e41e0-c3b4-7088-ab27-287cc2a2b190" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e41e0-c3b4-7cb3-9286-aa1ec1e6764d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7947-9bfe-2a484e3baf0b" xlink:to="loc_us-gaap_CommercialPaperMember_019e41e0-c3b4-7cb3-9286-aa1ec1e6764d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e41e0-c3b4-71d6-a2d3-a278d4724482" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b4-7947-9bfe-2a484e3baf0b" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e41e0-c3b4-71d6-a2d3-a278d4724482" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019e41e0-c3b4-7cc8-82e8-8a7aaa203833" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-73b5-b570-49021dbd30f8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019e41e0-c3b4-7cc8-82e8-8a7aaa203833" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_DebtSecuritiesAvailableForSaleFairValueMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughTwo_019e41e0-c3b4-7f23-a920-b4b30a895cbb" xlink:href="tndm-20260630.xsd#tndm_DebtSecuritiesAvailableForSaleFairValueMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-73b5-b570-49021dbd30f8" xlink:to="loc_tndm_DebtSecuritiesAvailableForSaleFairValueMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughTwo_019e41e0-c3b4-7f23-a920-b4b30a895cbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_DebtSecuritiesAvailableForSaleFairValueMaturityAllocatedAndSingleMaturityDateAfterYearTwoThroughThree_019e41e0-c3b4-7668-907e-3bee111a163f" xlink:href="tndm-20260630.xsd#tndm_DebtSecuritiesAvailableForSaleFairValueMaturityAllocatedAndSingleMaturityDateAfterYearTwoThroughThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-73b5-b570-49021dbd30f8" xlink:to="loc_tndm_DebtSecuritiesAvailableForSaleFairValueMaturityAllocatedAndSingleMaturityDateAfterYearTwoThroughThree_019e41e0-c3b4-7668-907e-3bee111a163f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate_019e41e0-c3b4-7fa1-a458-714d284251a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_CashCashEquivalentsAndShortTermInvestmentsLineItems_019e41e0-c3b4-73b5-b570-49021dbd30f8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate_019e41e0-c3b4-7fa1-a458-714d284251a7" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/FairValueMeasurementsScheduleofFinancialAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#FairValueMeasurementsScheduleofFinancialAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/FairValueMeasurementsScheduleofFinancialAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e41e0-c3b5-74f7-b78a-f9d4d3beefc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e41e0-c3b5-74f7-b78a-f9d4d3beefc0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019e41e0-c3b5-7976-979d-de1c8e225463" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e41e0-c3b5-74f7-b78a-f9d4d3beefc0" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019e41e0-c3b5-7976-979d-de1c8e225463" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e41e0-c3b5-7976-979d-de1c8e225463_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e41e0-c3b5-7976-979d-de1c8e225463" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e41e0-c3b5-7976-979d-de1c8e225463_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e41e0-c3b5-76d6-a4c4-15683fce4bc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e41e0-c3b5-7976-979d-de1c8e225463" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e41e0-c3b5-76d6-a4c4-15683fce4bc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsMember_019e41e0-c3b5-70be-b5ca-eb53e3d6f200" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e41e0-c3b5-76d6-a4c4-15683fce4bc0" xlink:to="loc_us-gaap_CashEquivalentsMember_019e41e0-c3b5-70be-b5ca-eb53e3d6f200" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-797a-bf35-44606a0a3252" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e41e0-c3b5-74f7-b78a-f9d4d3beefc0" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-797a-bf35-44606a0a3252" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-797a-bf35-44606a0a3252_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-797a-bf35-44606a0a3252" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-797a-bf35-44606a0a3252_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7ac5-91ec-386a255a747f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-797a-bf35-44606a0a3252" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7ac5-91ec-386a255a747f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e41e0-c3b5-755a-9854-ae47360c26b8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7ac5-91ec-386a255a747f" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e41e0-c3b5-755a-9854-ae47360c26b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e41e0-c3b5-7538-a323-acf305b007c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7ac5-91ec-386a255a747f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e41e0-c3b5-7538-a323-acf305b007c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e41e0-c3b5-7049-bb05-960491626f5e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7ac5-91ec-386a255a747f" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e41e0-c3b5-7049-bb05-960491626f5e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b5-7bd8-a018-1cd5c9cc58df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e41e0-c3b5-74f7-b78a-f9d4d3beefc0" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b5-7bd8-a018-1cd5c9cc58df" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7bd8-a018-1cd5c9cc58df_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b5-7bd8-a018-1cd5c9cc58df" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7bd8-a018-1cd5c9cc58df_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7227-a353-82dfc798873a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e41e0-c3b5-7bd8-a018-1cd5c9cc58df" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7227-a353-82dfc798873a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e41e0-c3b5-7fc5-9a36-d444985b7d10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7227-a353-82dfc798873a" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e41e0-c3b5-7fc5-9a36-d444985b7d10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e41e0-c3b5-7eae-a377-b4b1e09a8a35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7227-a353-82dfc798873a" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e41e0-c3b5-7eae-a377-b4b1e09a8a35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e41e0-c3b5-74fa-b2ee-ffb529ee241f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7227-a353-82dfc798873a" xlink:to="loc_us-gaap_CommercialPaperMember_019e41e0-c3b5-74fa-b2ee-ffb529ee241f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e41e0-c3b5-7d54-b0ac-e392debec2a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e41e0-c3b5-7227-a353-82dfc798873a" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e41e0-c3b5-7d54-b0ac-e392debec2a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c3b5-7c20-a1b8-1fd661693a24" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e41e0-c3b5-74f7-b78a-f9d4d3beefc0" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c3b5-7c20-a1b8-1fd661693a24" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c3b5-7c20-a1b8-1fd661693a24_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c3b5-7c20-a1b8-1fd661693a24" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c3b5-7c20-a1b8-1fd661693a24_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c3b5-7857-bef6-d99d26f5f5bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c3b5-7c20-a1b8-1fd661693a24" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c3b5-7857-bef6-d99d26f5f5bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeForwardMember_019e41e0-c3b5-7bf6-86e4-e6a52feff4be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignExchangeForwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c3b5-7857-bef6-d99d26f5f5bc" xlink:to="loc_us-gaap_ForeignExchangeForwardMember_019e41e0-c3b5-7bf6-86e4-e6a52feff4be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e41e0-c3b5-733f-9f9d-cec6b0c10c10" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e41e0-c3b5-74f7-b78a-f9d4d3beefc0" xlink:to="loc_srt_RangeAxis_019e41e0-c3b5-733f-9f9d-cec6b0c10c10" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e41e0-c3b5-733f-9f9d-cec6b0c10c10_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e41e0-c3b5-733f-9f9d-cec6b0c10c10" xlink:to="loc_srt_RangeMember_019e41e0-c3b5-733f-9f9d-cec6b0c10c10_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e41e0-c3b5-7755-a813-53b3fb7990b1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e41e0-c3b5-733f-9f9d-cec6b0c10c10" xlink:to="loc_srt_RangeMember_019e41e0-c3b5-7755-a813-53b3fb7990b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e41e0-c3b5-76ee-8a69-ff8d4c75d749" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e41e0-c3b5-7755-a813-53b3fb7990b1" xlink:to="loc_srt_MaximumMember_019e41e0-c3b5-76ee-8a69-ff8d4c75d749" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e41e0-c3b5-7e2c-8a8b-b9b7e5f2b649" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e41e0-c3b5-7e2c-8a8b-b9b7e5f2b649" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e41e0-c3b5-758e-b88a-9036fe5d3949" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e41e0-c3b5-758e-b88a-9036fe5d3949" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_019e41e0-c3b5-70e0-aee2-3b9e1193265c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_us-gaap_DerivativeAssets_019e41e0-c3b5-70e0-aee2-3b9e1193265c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019e41e0-c3b5-72d1-858f-9b008c208211" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019e41e0-c3b5-72d1-858f-9b008c208211" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_019e41e0-c3b5-7436-b392-a0172548bdb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_us-gaap_DerivativeLiabilities_019e41e0-c3b5-7436-b392-a0172548bdb9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_019e41e0-c3b5-73da-894b-19fc0d778002" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_019e41e0-c3b5-73da-894b-19fc0d778002" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_CashEquivalentsMaturityTerm_019e41e0-c3b5-73cf-99a3-eea4b4ab1ced" xlink:href="tndm-20260630.xsd#tndm_CashEquivalentsMaturityTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e41e0-c3b5-7ba7-96a9-15a06d4d63b4" xlink:to="loc_tndm_CashEquivalentsMaturityTerm_019e41e0-c3b5-73cf-99a3-eea4b4ab1ced" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/FairValueMeasurementsScheduleofEstimatedtheFairValueDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#FairValueMeasurementsScheduleofEstimatedtheFairValueDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/FairValueMeasurementsScheduleofEstimatedtheFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e41e0-c3b5-79f7-a917-38cb3da9e0d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e41e0-c3b5-7177-97f5-ffc3737966a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e41e0-c3b5-79f7-a917-38cb3da9e0d5" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e41e0-c3b5-7177-97f5-ffc3737966a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7b44-96ff-a7aa30de0ce3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e41e0-c3b5-7177-97f5-ffc3737966a5" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7b44-96ff-a7aa30de0ce3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7b44-96ff-a7aa30de0ce3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7b44-96ff-a7aa30de0ce3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7b44-96ff-a7aa30de0ce3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-74f4-89fc-653ecf245d90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7b44-96ff-a7aa30de0ce3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-74f4-89fc-653ecf245d90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-7e27-8cd7-9ccdb94ae2d7" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-74f4-89fc-653ecf245d90" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-7e27-8cd7-9ccdb94ae2d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2032Member_019e41e0-c3b5-7b81-b95c-e4afa9ce11e8" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-74f4-89fc-653ecf245d90" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2032Member_019e41e0-c3b5-7b81-b95c-e4afa9ce11e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7dcb-8f42-c6e1d0d0f4cd" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-74f4-89fc-653ecf245d90" xlink:to="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7dcb-8f42-c6e1d0d0f4cd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-7017-98d8-0b00957dc809" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e41e0-c3b5-7177-97f5-ffc3737966a5" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-7017-98d8-0b00957dc809" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7017-98d8-0b00957dc809_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-7017-98d8-0b00957dc809" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7017-98d8-0b00957dc809_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7212-8947-9fa9c26f69f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e41e0-c3b5-7017-98d8-0b00957dc809" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7212-8947-9fa9c26f69f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e41e0-c3b5-7a3e-a49e-f5959200a934" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e41e0-c3b5-7212-8947-9fa9c26f69f5" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e41e0-c3b5-7a3e-a49e-f5959200a934" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7727-a3ff-5321194cf101" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019e41e0-c3b5-7177-97f5-ffc3737966a5" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7727-a3ff-5321194cf101" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7727-a3ff-5321194cf101_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7727-a3ff-5321194cf101" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7727-a3ff-5321194cf101_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-726e-b2c9-d62430b8dcf5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7727-a3ff-5321194cf101" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-726e-b2c9-d62430b8dcf5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7484-898a-4ed5cd450e9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-726e-b2c9-d62430b8dcf5" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7484-898a-4ed5cd450e9e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_019e41e0-c3b5-7a70-9a6b-be08a88b6631" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019e41e0-c3b5-79f7-a917-38cb3da9e0d5" xlink:to="loc_us-gaap_LongTermDebtFairValue_019e41e0-c3b5-7a70-9a6b-be08a88b6631" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/LeasesAdditionalInformationDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#LeasesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/LeasesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e41e0-c3b5-703f-86b0-e7ebcfa225e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019e41e0-c3b5-713c-aff4-3541a1659bfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e41e0-c3b5-703f-86b0-e7ebcfa225e3" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019e41e0-c3b5-713c-aff4-3541a1659bfb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e41e0-c3b5-70b6-8a16-2857cd853ad0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e41e0-c3b5-713c-aff4-3541a1659bfb" xlink:to="loc_srt_RangeAxis_019e41e0-c3b5-70b6-8a16-2857cd853ad0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e41e0-c3b5-70b6-8a16-2857cd853ad0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e41e0-c3b5-70b6-8a16-2857cd853ad0" xlink:to="loc_srt_RangeMember_019e41e0-c3b5-70b6-8a16-2857cd853ad0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e41e0-c3b5-7fc4-a48a-6cb9be397c57" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e41e0-c3b5-70b6-8a16-2857cd853ad0" xlink:to="loc_srt_RangeMember_019e41e0-c3b5-7fc4-a48a-6cb9be397c57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e41e0-c3b5-736c-81fc-51c1f160f99a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e41e0-c3b5-7fc4-a48a-6cb9be397c57" xlink:to="loc_srt_MinimumMember_019e41e0-c3b5-736c-81fc-51c1f160f99a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e41e0-c3b5-7e16-add0-c73f61fee53c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e41e0-c3b5-7fc4-a48a-6cb9be397c57" xlink:to="loc_srt_MaximumMember_019e41e0-c3b5-7e16-add0-c73f61fee53c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermAxis_019e41e0-c3b5-702b-b118-c9a4d21f0a30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseContractualTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e41e0-c3b5-713c-aff4-3541a1659bfb" xlink:to="loc_us-gaap_LeaseContractualTermAxis_019e41e0-c3b5-702b-b118-c9a4d21f0a30" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_019e41e0-c3b5-702b-b118-c9a4d21f0a30_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseContractualTermAxis_019e41e0-c3b5-702b-b118-c9a4d21f0a30" xlink:to="loc_us-gaap_LeaseContractualTermDomain_019e41e0-c3b5-702b-b118-c9a4d21f0a30_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_019e41e0-c3b5-70ad-ac13-1372761c9f93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseContractualTermAxis_019e41e0-c3b5-702b-b118-c9a4d21f0a30" xlink:to="loc_us-gaap_LeaseContractualTermDomain_019e41e0-c3b5-70ad-ac13-1372761c9f93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_HeadquartersLeaseMember_019f11ce-66a8-7988-b16c-b9e4987a9b18" xlink:href="tndm-20260630.xsd#tndm_HeadquartersLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_019e41e0-c3b5-70ad-ac13-1372761c9f93" xlink:to="loc_tndm_HeadquartersLeaseMember_019f11ce-66a8-7988-b16c-b9e4987a9b18" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_HeadquartersLeasePhaseIIMember_019e41e0-c3b5-7866-9fcb-8eda84d20299" xlink:href="tndm-20260630.xsd#tndm_HeadquartersLeasePhaseIIMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_019e41e0-c3b5-70ad-ac13-1372761c9f93" xlink:to="loc_tndm_HeadquartersLeasePhaseIIMember_019e41e0-c3b5-7866-9fcb-8eda84d20299" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e41e0-c3b5-7508-ad8e-283edcfa6074" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e41e0-c3b5-713c-aff4-3541a1659bfb" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e41e0-c3b5-7508-ad8e-283edcfa6074" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e41e0-c3b5-7508-ad8e-283edcfa6074_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e41e0-c3b5-7508-ad8e-283edcfa6074" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e41e0-c3b5-7508-ad8e-283edcfa6074_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e41e0-c3b5-76d5-b156-624059ede8a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e41e0-c3b5-7508-ad8e-283edcfa6074" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e41e0-c3b5-76d5-b156-624059ede8a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseAgreementsMember_019e41e0-c3b5-75c8-91d5-43c640c5a77b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e41e0-c3b5-76d5-b156-624059ede8a1" xlink:to="loc_us-gaap_LeaseAgreementsMember_019e41e0-c3b5-75c8-91d5-43c640c5a77b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019e41e0-c3b5-78ef-8e6e-3e2130d6e222" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e41e0-c3b5-703f-86b0-e7ebcfa225e3" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019e41e0-c3b5-78ef-8e6e-3e2130d6e222" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_019f11ce-66a8-7b1c-80fa-29fdb7d74b33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e41e0-c3b5-703f-86b0-e7ebcfa225e3" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_019f11ce-66a8-7b1c-80fa-29fdb7d74b33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_019e41e0-c3b5-77ac-9043-3a962ee6707b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e41e0-c3b5-703f-86b0-e7ebcfa225e3" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_019e41e0-c3b5-77ac-9043-3a962ee6707b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/DebtAdditionalInformationDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#DebtAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/DebtAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-76cb-a82f-547f66645684" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-76cb-a82f-547f66645684" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7caf-9c10-7f853678d097" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-76cb-a82f-547f66645684" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7caf-9c10-7f853678d097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7caf-9c10-7f853678d097_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7caf-9c10-7f853678d097" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7caf-9c10-7f853678d097_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-78cb-9601-c069eb5e5680" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7caf-9c10-7f853678d097" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-78cb-9601-c069eb5e5680" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2025Member_019e41e0-c3b5-7f9a-8136-605b399daad5" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-78cb-9601-c069eb5e5680" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2025Member_019e41e0-c3b5-7f9a-8136-605b399daad5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-796c-a94a-ace135a9b542" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-78cb-9601-c069eb5e5680" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-796c-a94a-ace135a9b542" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2032Member_019e41e0-c3b5-7e13-aa72-559853429c96" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-78cb-9601-c069eb5e5680" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2032Member_019e41e0-c3b5-7e13-aa72-559853429c96" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-743a-a733-7a79b48776e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-76cb-a82f-547f66645684" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-743a-a733-7a79b48776e2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-743a-a733-7a79b48776e2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-743a-a733-7a79b48776e2" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-743a-a733-7a79b48776e2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-741c-b6d8-000778341e25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-743a-a733-7a79b48776e2" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-741c-b6d8-000778341e25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7f3c-a132-e666133f001b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-741c-b6d8-000778341e25" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7f3c-a132-e666133f001b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e41e0-c3b5-7e7b-8fa6-149d2ce503c5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-76cb-a82f-547f66645684" xlink:to="loc_srt_RangeAxis_019e41e0-c3b5-7e7b-8fa6-149d2ce503c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e41e0-c3b5-7e7b-8fa6-149d2ce503c5_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e41e0-c3b5-7e7b-8fa6-149d2ce503c5" xlink:to="loc_srt_RangeMember_019e41e0-c3b5-7e7b-8fa6-149d2ce503c5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e41e0-c3b5-76d4-b8ed-24753b9df70c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e41e0-c3b5-7e7b-8fa6-149d2ce503c5" xlink:to="loc_srt_RangeMember_019e41e0-c3b5-76d4-b8ed-24753b9df70c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e41e0-c3b5-7996-95f6-0d85d6a85f75" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e41e0-c3b5-76d4-b8ed-24753b9df70c" xlink:to="loc_srt_MinimumMember_019e41e0-c3b5-7996-95f6-0d85d6a85f75" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtConversionByUniqueDescriptionAxis_019e41e0-c3b5-7cc0-91fe-1164bd5c656a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtConversionByUniqueDescriptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-76cb-a82f-547f66645684" xlink:to="loc_us-gaap_DebtConversionByUniqueDescriptionAxis_019e41e0-c3b5-7cc0-91fe-1164bd5c656a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtConversionNameDomain_019e41e0-c3b5-7cc0-91fe-1164bd5c656a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtConversionNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtConversionByUniqueDescriptionAxis_019e41e0-c3b5-7cc0-91fe-1164bd5c656a" xlink:to="loc_us-gaap_DebtConversionNameDomain_019e41e0-c3b5-7cc0-91fe-1164bd5c656a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtConversionNameDomain_019e41e0-c3b5-7153-898a-186292a1c744" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtConversionNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtConversionByUniqueDescriptionAxis_019e41e0-c3b5-7cc0-91fe-1164bd5c656a" xlink:to="loc_us-gaap_DebtConversionNameDomain_019e41e0-c3b5-7153-898a-186292a1c744" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConversionInstance130Member_019e41e0-c3b5-72a6-9730-2142b9b3b32a" xlink:href="tndm-20260630.xsd#tndm_ConversionInstance130Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtConversionNameDomain_019e41e0-c3b5-7153-898a-186292a1c744" xlink:to="loc_tndm_ConversionInstance130Member_019e41e0-c3b5-72a6-9730-2142b9b3b32a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConversionInstance98Member_019e41e0-c3b5-7a5a-b9c1-dab45da83d4d" xlink:href="tndm-20260630.xsd#tndm_ConversionInstance98Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtConversionNameDomain_019e41e0-c3b5-7153-898a-186292a1c744" xlink:to="loc_tndm_ConversionInstance98Member_019e41e0-c3b5-7a5a-b9c1-dab45da83d4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e41e0-c3b5-72f8-81c1-42ef14ce8763" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e41e0-c3b5-72f8-81c1-42ef14ce8763" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtExtinguishmentCosts_019e41e0-c3b5-77a3-bbc9-00ed5d95a580" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfDebtExtinguishmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_PaymentsOfDebtExtinguishmentCosts_019e41e0-c3b5-77a3-bbc9-00ed5d95a580" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e41e0-c3b5-7009-b0b8-a72730f0fdab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e41e0-c3b5-7009-b0b8-a72730f0fdab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfDebt_019e41e0-c3b5-7e55-9d82-aace043263c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfDebt_019e41e0-c3b5-7e55-9d82-aace043263c9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e41e0-c3b5-7ce4-ad0f-6185d7899037" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e41e0-c3b5-7ce4-ad0f-6185d7899037" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019e41e0-c3b5-7eb2-9fba-3d04f806dea8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019e41e0-c3b5-7eb2-9fba-3d04f806dea8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019e41e0-c3b5-7b6c-baf5-9db5f5e37d0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019e41e0-c3b5-7b6c-baf5-9db5f5e37d0e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019e41e0-c3b5-7dea-aa6f-9bb49c53b734" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019e41e0-c3b5-7dea-aa6f-9bb49c53b734" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019e41e0-c3b5-7c4f-b4ee-38770e7c188c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019e41e0-c3b5-7c4f-b4ee-38770e7c188c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019e41e0-c3b5-7b6c-bbba-a1e27fa99aee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019e41e0-c3b5-7b6c-bbba-a1e27fa99aee" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_DebtInstrumentAdditionalPrincipalAmount_019e41e0-c3b5-76f6-aa0a-9fa916031e9f" xlink:href="tndm-20260630.xsd#tndm_DebtInstrumentAdditionalPrincipalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_tndm_DebtInstrumentAdditionalPrincipalAmount_019e41e0-c3b5-76f6-aa0a-9fa916031e9f" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromConvertibleDebt_019e41e0-c3b5-71cb-b56d-dc2ddfb02c7e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_ProceedsFromConvertibleDebt_019e41e0-c3b5-71cb-b56d-dc2ddfb02c7e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_019e41e0-c3b5-7632-91c6-87b49bcfeab6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtConversionConvertedInstrumentAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_019e41e0-c3b5-7632-91c6-87b49bcfeab6" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_DebtInstrumentCappedCallNetCost_019e41e0-c3b5-7276-96a3-d8436f718919" xlink:href="tndm-20260630.xsd#tndm_DebtInstrumentCappedCallNetCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_tndm_DebtInstrumentCappedCallNetCost_019e41e0-c3b5-7276-96a3-d8436f718919" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_OptionIndexedToIssuersEquityCapPrice_019e41e0-c3b5-7015-a68e-35bd942e73fd" xlink:href="tndm-20260630.xsd#tndm_OptionIndexedToIssuersEquityCapPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-78ad-b083-64e3b350b75e" xlink:to="loc_tndm_OptionIndexedToIssuersEquityCapPrice_019e41e0-c3b5-7015-a68e-35bd942e73fd" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/DebtScheduleofComponentsofConvertibleNoteDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#DebtScheduleofComponentsofConvertibleNoteDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/DebtScheduleofComponentsofConvertibleNoteDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7806-bc1b-ad36ebe8928f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-762f-b4e1-69cbe9d6056f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7806-bc1b-ad36ebe8928f" xlink:to="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-762f-b4e1-69cbe9d6056f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-762a-9bbb-736924078a4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-762f-b4e1-69cbe9d6056f" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-762a-9bbb-736924078a4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-762a-9bbb-736924078a4a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-762a-9bbb-736924078a4a" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-762a-9bbb-736924078a4a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7908-8fb0-a6b653fb01f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-762a-9bbb-736924078a4a" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7908-8fb0-a6b653fb01f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7cdf-893f-8678437f5a0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7908-8fb0-a6b653fb01f0" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7cdf-893f-8678437f5a0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-794c-ac60-13cc7c471daf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-762f-b4e1-69cbe9d6056f" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-794c-ac60-13cc7c471daf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-794c-ac60-13cc7c471daf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-794c-ac60-13cc7c471daf" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-794c-ac60-13cc7c471daf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7495-a6d0-09a44260819b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-794c-ac60-13cc7c471daf" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7495-a6d0-09a44260819b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7ed1-844e-bade8751dc21" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7495-a6d0-09a44260819b" xlink:to="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7ed1-844e-bade8751dc21" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-7677-8f22-290e556a147a" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7ed1-844e-bade8751dc21" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-7677-8f22-290e556a147a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2032Member_019f11e1-1a74-78e8-89e1-72c79837edc3" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7ed1-844e-bade8751dc21" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2032Member_019f11e1-1a74-78e8-89e1-72c79837edc3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e41e0-c3b5-72c7-9b17-8852d5c6dde1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7806-bc1b-ad36ebe8928f" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e41e0-c3b5-72c7-9b17-8852d5c6dde1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019e41e0-c3b5-76dd-9068-2d9736c496d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7806-bc1b-ad36ebe8928f" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019e41e0-c3b5-76dd-9068-2d9736c496d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019e41e0-c3b5-74cb-90f3-55e5f3a6f5de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7806-bc1b-ad36ebe8928f" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019e41e0-c3b5-74cb-90f3-55e5f3a6f5de" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/DebtScheduleofEffectiveInterestRatesofConvertibleSeniorNotesDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#DebtScheduleofEffectiveInterestRatesofConvertibleSeniorNotesDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/DebtScheduleofEffectiveInterestRatesofConvertibleSeniorNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7f22-ae30-f8a2b58aa227" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7745-9c5b-f1d0dafdb62a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7f22-ae30-f8a2b58aa227" xlink:to="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7745-9c5b-f1d0dafdb62a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7dc4-9693-30984fd4fb30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7745-9c5b-f1d0dafdb62a" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7dc4-9693-30984fd4fb30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7dc4-9693-30984fd4fb30_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7dc4-9693-30984fd4fb30" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7dc4-9693-30984fd4fb30_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7c5e-a16d-f4930c6257a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-7dc4-9693-30984fd4fb30" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7c5e-a16d-f4930c6257a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2025Member_019e41e0-c3b5-7196-b815-1f5a2bdab7c5" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7c5e-a16d-f4930c6257a1" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2025Member_019e41e0-c3b5-7196-b815-1f5a2bdab7c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-7ffe-afb0-bd127abf04cc" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7c5e-a16d-f4930c6257a1" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2029Member_019e41e0-c3b5-7ffe-afb0-bd127abf04cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesDue2032Member_019e41e0-c3b5-706d-b7a6-2027994a7f00" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesDue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7c5e-a16d-f4930c6257a1" xlink:to="loc_tndm_ConvertibleSeniorNotesDue2032Member_019e41e0-c3b5-706d-b7a6-2027994a7f00" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b9a-b85f-b5eac0081323" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7745-9c5b-f1d0dafdb62a" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b9a-b85f-b5eac0081323" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7b9a-b85f-b5eac0081323_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b9a-b85f-b5eac0081323" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7b9a-b85f-b5eac0081323_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7931-86e4-af4b85f36e18" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b9a-b85f-b5eac0081323" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7931-86e4-af4b85f36e18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-74f1-be2f-cca876cef723" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7931-86e4-af4b85f36e18" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-74f1-be2f-cca876cef723" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e41e0-c3b5-7e0a-9775-657027b33baf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-7f22-ae30-f8a2b58aa227" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e41e0-c3b5-7e0a-9775-657027b33baf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/DebtScheduleofInterestExpenseRecognizedDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#DebtScheduleofInterestExpenseRecognizedDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/DebtScheduleofInterestExpenseRecognizedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-76bc-9a62-1aae12d7cb84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7eae-801d-3385ffa63929" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-76bc-9a62-1aae12d7cb84" xlink:to="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7eae-801d-3385ffa63929" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b1c-82b3-2f4dde0455a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7eae-801d-3385ffa63929" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b1c-82b3-2f4dde0455a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7b1c-82b3-2f4dde0455a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b1c-82b3-2f4dde0455a8" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-7b1c-82b3-2f4dde0455a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-78e6-9ba9-9e02ee680524" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e41e0-c3b5-7b1c-82b3-2f4dde0455a8" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-78e6-9ba9-9e02ee680524" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7002-86f5-516a7496898b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e41e0-c3b5-78e6-9ba9-9e02ee680524" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e41e0-c3b5-7002-86f5-516a7496898b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-72fb-b7f4-ae9c1f76d01d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e41e0-c3b5-7eae-801d-3385ffa63929" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-72fb-b7f4-ae9c1f76d01d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-72fb-b7f4-ae9c1f76d01d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-72fb-b7f4-ae9c1f76d01d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-72fb-b7f4-ae9c1f76d01d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7db3-ae81-12b6be7da934" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e41e0-c3b5-72fb-b7f4-ae9c1f76d01d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7db3-ae81-12b6be7da934" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7aa6-8d2a-a1f2d308fd15" xlink:href="tndm-20260630.xsd#tndm_ConvertibleSeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e41e0-c3b5-7db3-ae81-12b6be7da934" xlink:to="loc_tndm_ConvertibleSeniorNotesMember_019e41e0-c3b5-7aa6-8d2a-a1f2d308fd15" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019e41e0-c3b5-785a-81bc-9abe46e9dc3f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-76bc-9a62-1aae12d7cb84" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019e41e0-c3b5-785a-81bc-9abe46e9dc3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_019e41e0-c3b5-7c05-bb2c-484b2f2c5eff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-76bc-9a62-1aae12d7cb84" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_019e41e0-c3b5-7c05-bb2c-484b2f2c5eff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019e41e0-c3b5-710f-bde5-85f28264ff00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e41e0-c3b5-76bc-9a62-1aae12d7cb84" xlink:to="loc_us-gaap_InterestExpenseDebt_019e41e0-c3b5-710f-bde5-85f28264ff00" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofSharesofCommonStockReservedforFutureIssuanceDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#StockholdersEquityScheduleofSharesofCommonStockReservedforFutureIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofSharesofCommonStockReservedforFutureIssuanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7c12-80bf-cf46961b4fe6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7b5c-8798-b484695bb5cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7c12-80bf-cf46961b4fe6" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7b5c-8798-b484695bb5cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c46c-7cf5-bd08-011696b7c6ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7b5c-8798-b484695bb5cb" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c46c-7cf5-bd08-011696b7c6ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7cf5-bd08-011696b7c6ce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c46c-7cf5-bd08-011696b7c6ce" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7cf5-bd08-011696b7c6ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e41e0-c46c-7cf5-bd08-011696b7c6ce" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_019e41e0-c46c-79e1-87f3-153cbcb37a49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_019e41e0-c46c-79e1-87f3-153cbcb37a49" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_019e41e0-c46c-7865-9e0e-00530ec1b2f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:to="loc_us-gaap_WarrantMember_019e41e0-c46c-7865-9e0e-00530ec1b2f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_StockOptionsIssuedAndOutstandingMember_019e41e0-c46c-73dc-97db-c5c4093c0eda" xlink:href="tndm-20260630.xsd#tndm_StockOptionsIssuedAndOutstandingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:to="loc_tndm_StockOptionsIssuedAndOutstandingMember_019e41e0-c46c-73dc-97db-c5c4093c0eda" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e41e0-c46c-71e3-97ad-969c6a4cf743" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e41e0-c46c-71e3-97ad-969c6a4cf743" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_019e41e0-c46c-75e6-a9f6-6ce284d102ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:to="loc_us-gaap_StockCompensationPlanMember_019e41e0-c46c-75e6-a9f6-6ce284d102ae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_AuthorizedForFutureOptionGrantsMember_019e41e0-c46c-7559-b747-0c4b0ea2b800" xlink:href="tndm-20260630.xsd#tndm_AuthorizedForFutureOptionGrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e41e0-c46c-7c37-9dda-b79d180f3411" xlink:to="loc_tndm_AuthorizedForFutureOptionGrantsMember_019e41e0-c46c-7559-b747-0c4b0ea2b800" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e41e0-c46c-7816-9f16-fdc50373d878" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7c12-80bf-cf46961b4fe6" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e41e0-c46c-7816-9f16-fdc50373d878" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofCommonStockWarrantsDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#StockholdersEquityScheduleofCommonStockWarrantsDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofCommonStockWarrantsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7a16-820e-77a28f8570dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-77a8-ab0a-d22d71beaaa1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7a16-820e-77a28f8570dc" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-77a8-ab0a-d22d71beaaa1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_019e41e0-c46c-7427-b3c1-43d3b5306b84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-77a8-ab0a-d22d71beaaa1" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_019e41e0-c46c-7427-b3c1-43d3b5306b84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019e41e0-c46c-7427-b3c1-43d3b5306b84_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019e41e0-c46c-7427-b3c1-43d3b5306b84" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019e41e0-c46c-7427-b3c1-43d3b5306b84_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019e41e0-c46c-7291-a94c-773c0aca435e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019e41e0-c46c-7427-b3c1-43d3b5306b84" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019e41e0-c46c-7291-a94c-773c0aca435e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_CommonStockWarrantExpiringMarch2027Member_019e41e0-c46c-7e4a-bce0-7253a0007cf7" xlink:href="tndm-20260630.xsd#tndm_CommonStockWarrantExpiringMarch2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019e41e0-c46c-7291-a94c-773c0aca435e" xlink:to="loc_tndm_CommonStockWarrantExpiringMarch2027Member_019e41e0-c46c-7e4a-bce0-7253a0007cf7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_019e41e0-c46c-7cba-85ea-3d6efd1fb3a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7a16-820e-77a28f8570dc" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_019e41e0-c46c-7cba-85ea-3d6efd1fb3a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019e41e0-c46c-7b53-b40f-c8ea5a5fe736" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7a16-820e-77a28f8570dc" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019e41e0-c46c-7b53-b40f-c8ea5a5fe736" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofAllocationofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#StockholdersEquityScheduleofAllocationofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofAllocationofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-794e-ab78-6b272104baab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7fe7-92fb-d7dfe7a152d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-794e-ab78-6b272104baab" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7fe7-92fb-d7dfe7a152d3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e41e0-c46c-7bbd-8c16-4f03be2d5c81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7fe7-92fb-d7dfe7a152d3" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e41e0-c46c-7bbd-8c16-4f03be2d5c81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e41e0-c46c-7671-83f6-0d60f6e81cf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-794e-ab78-6b272104baab" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e41e0-c46c-7671-83f6-0d60f6e81cf7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofAllocationofStockBasedCompensationExpenseDetails_1" xlink:type="simple" xlink:href="tndm-20260630.xsd#StockholdersEquityScheduleofAllocationofStockBasedCompensationExpenseDetails_1"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofAllocationofStockBasedCompensationExpenseDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-794e-ab78-6b272104baab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7fe7-92fb-d7dfe7a152d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-794e-ab78-6b272104baab" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7fe7-92fb-d7dfe7a152d3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e41e0-c46c-7671-83f6-0d60f6e81cf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-794e-ab78-6b272104baab" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e41e0-c46c-7671-83f6-0d60f6e81cf7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofRestrictedStockUnitsRSUsDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#StockholdersEquityScheduleofRestrictedStockUnitsRSUsDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofRestrictedStockUnitsRSUsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-750b-ac2e-4b2ad0da7caa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7b9d-98f7-1dcb3980cb9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-750b-ac2e-4b2ad0da7caa" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7b9d-98f7-1dcb3980cb9b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7930-bd7d-86f1c0f07dc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7b9d-98f7-1dcb3980cb9b" xlink:to="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7930-bd7d-86f1c0f07dc9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-7930-bd7d-86f1c0f07dc9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7930-bd7d-86f1c0f07dc9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-7930-bd7d-86f1c0f07dc9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-78ec-be06-ee3604bac65b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7930-bd7d-86f1c0f07dc9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-78ec-be06-ee3604bac65b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e41e0-c46c-7894-8c71-fab7ff771ec5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-78ec-be06-ee3604bac65b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e41e0-c46c-7894-8c71-fab7ff771ec5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e41e0-c46c-7c69-bd98-02d4b770f745" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-750b-ac2e-4b2ad0da7caa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e41e0-c46c-7c69-bd98-02d4b770f745" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e41e0-c46c-7990-87a6-55e456407035" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-750b-ac2e-4b2ad0da7caa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e41e0-c46c-7990-87a6-55e456407035" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofAssumptionsUsedinBlackScholesOptionPricingModelDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#StockholdersEquityScheduleofAssumptionsUsedinBlackScholesOptionPricingModelDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/StockholdersEquityScheduleofAssumptionsUsedinBlackScholesOptionPricingModelDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7fc5-9d5e-f49ed876c50a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7ffc-b93a-c87b6ae09be9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7fc5-9d5e-f49ed876c50a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7ffc-b93a-c87b6ae09be9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7d43-b8f4-69c0e594d18a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e41e0-c46c-7ffc-b93a-c87b6ae09be9" xlink:to="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7d43-b8f4-69c0e594d18a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-7d43-b8f4-69c0e594d18a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7d43-b8f4-69c0e594d18a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-7d43-b8f4-69c0e594d18a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-7c38-92fd-19dc1f375ce3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e41e0-c46c-7d43-b8f4-69c0e594d18a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-7c38-92fd-19dc1f375ce3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e41e0-c46c-73dd-a0d5-742affe5f40b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e41e0-c46c-7c38-92fd-19dc1f375ce3" xlink:to="loc_us-gaap_EmployeeStockMember_019e41e0-c46c-73dd-a0d5-742affe5f40b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e41e0-c46c-7af1-bb99-f886b6e42be0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7fc5-9d5e-f49ed876c50a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e41e0-c46c-7af1-bb99-f886b6e42be0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e41e0-c46c-7d85-9ff7-c147c2510c2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7fc5-9d5e-f49ed876c50a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e41e0-c46c-7d85-9ff7-c147c2510c2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e41e0-c46c-7c9a-924c-e633e66ca7f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7fc5-9d5e-f49ed876c50a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e41e0-c46c-7c9a-924c-e633e66ca7f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e41e0-c46c-7649-a0f9-71684b1170be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7fc5-9d5e-f49ed876c50a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e41e0-c46c-7649-a0f9-71684b1170be" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e41e0-c46c-7264-8134-5ffb59159cc8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e41e0-c46c-7fc5-9d5e-f49ed876c50a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e41e0-c46c-7264-8134-5ffb59159cc8" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/DerivativesAdditionalInformationDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#DerivativesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/DerivativesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7318-8d51-872cb68985d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e41e0-c46c-76de-a1c3-3aab3c981515" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7318-8d51-872cb68985d5" xlink:to="loc_us-gaap_DerivativeTable_019e41e0-c46c-76de-a1c3-3aab3c981515" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-77b8-9af6-71325cbcbca4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e41e0-c46c-76de-a1c3-3aab3c981515" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-77b8-9af6-71325cbcbca4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-77b8-9af6-71325cbcbca4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-77b8-9af6-71325cbcbca4" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-77b8-9af6-71325cbcbca4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-73d2-98cb-445e5a2a7261" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-77b8-9af6-71325cbcbca4" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-73d2-98cb-445e5a2a7261" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e41e0-c46c-7760-aefb-086c6dd79ac8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-73d2-98cb-445e5a2a7261" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e41e0-c46c-7760-aefb-086c6dd79ac8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-75cd-b005-cd25f91f4e5e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e41e0-c46c-76de-a1c3-3aab3c981515" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-75cd-b005-cd25f91f4e5e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-75cd-b005-cd25f91f4e5e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-75cd-b005-cd25f91f4e5e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-75cd-b005-cd25f91f4e5e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-77f2-8fc3-967c5ca14fce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-75cd-b005-cd25f91f4e5e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-77f2-8fc3-967c5ca14fce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeForwardMember_019e41e0-c46c-75f6-a84b-fe4d5e8f9ad5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignExchangeForwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-77f2-8fc3-967c5ca14fce" xlink:to="loc_us-gaap_ForeignExchangeForwardMember_019e41e0-c46c-75f6-a84b-fe4d5e8f9ad5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019e41e0-c46c-7012-b8a6-694b74694504" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7318-8d51-872cb68985d5" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019e41e0-c46c-7012-b8a6-694b74694504" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_UnrealizedGainOnDerivatives_019e41e0-c46c-74d8-b1f0-553b0477c686" xlink:href="tndm-20260630.xsd#tndm_UnrealizedGainOnDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7318-8d51-872cb68985d5" xlink:to="loc_tndm_UnrealizedGainOnDerivatives_019e41e0-c46c-74d8-b1f0-553b0477c686" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_UnrealizedLossOnDerivatives_019e41e0-c46c-77cb-b0df-6d9ed1466cd3" xlink:href="tndm-20260630.xsd#tndm_UnrealizedLossOnDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7318-8d51-872cb68985d5" xlink:to="loc_tndm_UnrealizedLossOnDerivatives_019e41e0-c46c-77cb-b0df-6d9ed1466cd3" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/DerivativesScheduleoftheEffectofForeignCurrencyForwardContractsDesignatedasHedgingInstrumentsDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#DerivativesScheduleoftheEffectofForeignCurrencyForwardContractsDesignatedasHedgingInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/DerivativesScheduleoftheEffectofForeignCurrencyForwardContractsDesignatedasHedgingInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7310-a3b1-ec1e0d8d055d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7310-a3b1-ec1e0d8d055d" xlink:to="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-70d5-bdd7-0a05cd945936_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-70d5-bdd7-0a05cd945936_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-7ab0-8da7-55909c6cd5d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-7ab0-8da7-55909c6cd5d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeContractMember_019e41e0-c46c-75a6-88d1-95bfccd3fd8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignExchangeContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-7ab0-8da7-55909c6cd5d0" xlink:to="loc_us-gaap_ForeignExchangeContractMember_019e41e0-c46c-75a6-88d1-95bfccd3fd8d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-78d7-b8f3-4f880fa3be59_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-78d7-b8f3-4f880fa3be59_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-7442-892a-76ec944cd6d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-7442-892a-76ec944cd6d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e41e0-c46c-74f2-a96b-4ff987f952dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-7442-892a-76ec944cd6d2" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e41e0-c46c-74f2-a96b-4ff987f952dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e41e0-c46c-7bfb-b2dd-dffb5e929769" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e41e0-c46c-7bfb-b2dd-dffb5e929769" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_019e41e0-c46c-78ae-9982-2cb085b46170" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7310-a3b1-ec1e0d8d055d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_019e41e0-c46c-78ae-9982-2cb085b46170" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/DerivativesScheduleoftheEffectofForeignCurrencyForwardContractsDesignatedasHedgingInstrumentsDetails_1" xlink:type="simple" xlink:href="tndm-20260630.xsd#DerivativesScheduleoftheEffectofForeignCurrencyForwardContractsDesignatedasHedgingInstrumentsDetails_1"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/DerivativesScheduleoftheEffectofForeignCurrencyForwardContractsDesignatedasHedgingInstrumentsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7310-a3b1-ec1e0d8d055d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7310-a3b1-ec1e0d8d055d" xlink:to="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-70d5-bdd7-0a05cd945936_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-70d5-bdd7-0a05cd945936_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-7ab0-8da7-55909c6cd5d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e41e0-c46c-70d5-bdd7-0a05cd945936" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-7ab0-8da7-55909c6cd5d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeContractMember_019e41e0-c46c-75a6-88d1-95bfccd3fd8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignExchangeContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e41e0-c46c-7ab0-8da7-55909c6cd5d0" xlink:to="loc_us-gaap_ForeignExchangeContractMember_019e41e0-c46c-75a6-88d1-95bfccd3fd8d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e41e0-c46c-77f9-8073-7344ae261a58" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-78d7-b8f3-4f880fa3be59_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-78d7-b8f3-4f880fa3be59_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-7442-892a-76ec944cd6d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e41e0-c46c-78d7-b8f3-4f880fa3be59" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-7442-892a-76ec944cd6d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e41e0-c46c-74f2-a96b-4ff987f952dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e41e0-c46c-7442-892a-76ec944cd6d2" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019e41e0-c46c-74f2-a96b-4ff987f952dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_019e41e0-c46c-78ae-9982-2cb085b46170" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e41e0-c46c-7310-a3b1-ec1e0d8d055d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_019e41e0-c46c-78ae-9982-2cb085b46170" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/BusinessSegmentandGeographicInformationAdditionalInformationDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#BusinessSegmentandGeographicInformationAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/BusinessSegmentandGeographicInformationAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_019e41e0-c46c-7652-a110-6a18492d5f58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_019e41e0-c46c-70cc-8df1-a782d451e634" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_019e41e0-c46c-7652-a110-6a18492d5f58" xlink:to="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_019e41e0-c46c-70cc-8df1-a782d451e634" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e41e0-c46c-77ef-96f5-f4c8c540d8aa" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_019e41e0-c46c-70cc-8df1-a782d451e634" xlink:to="loc_srt_StatementGeographicalAxis_019e41e0-c46c-77ef-96f5-f4c8c540d8aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-77ef-96f5-f4c8c540d8aa_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e41e0-c46c-77ef-96f5-f4c8c540d8aa" xlink:to="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-77ef-96f5-f4c8c540d8aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7498-9916-3c35373ce6cb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e41e0-c46c-77ef-96f5-f4c8c540d8aa" xlink:to="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7498-9916-3c35373ce6cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e41e0-c46c-7fc1-a9d9-1de5425021c3" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7498-9916-3c35373ce6cb" xlink:to="loc_country_US_019e41e0-c46c-7fc1-a9d9-1de5425021c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_019e41e0-c46c-7fc7-a8b0-b1d37739029f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7498-9916-3c35373ce6cb" xlink:to="loc_us-gaap_NonUsMember_019e41e0-c46c-7fc7-a8b0-b1d37739029f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e41e0-c46c-7ad2-842e-9e2a8996b9e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_019e41e0-c46c-70cc-8df1-a782d451e634" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e41e0-c46c-7ad2-842e-9e2a8996b9e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e41e0-c46c-7ad2-842e-9e2a8996b9e7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e41e0-c46c-7ad2-842e-9e2a8996b9e7" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e41e0-c46c-7ad2-842e-9e2a8996b9e7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e41e0-c46c-7c04-9348-bc2b60e2d621" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e41e0-c46c-7ad2-842e-9e2a8996b9e7" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e41e0-c46c-7c04-9348-bc2b60e2d621" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueProductLineMember_019e41e0-c46c-76c3-9498-6df55ebba0ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesRevenueProductLineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e41e0-c46c-7c04-9348-bc2b60e2d621" xlink:to="loc_us-gaap_SalesRevenueProductLineMember_019e41e0-c46c-76c3-9498-6df55ebba0ff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_019e41e0-c46c-77d1-acc6-a15d2c441e7e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e41e0-c46c-7c04-9348-bc2b60e2d621" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_019e41e0-c46c-77d1-acc6-a15d2c441e7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019e41e0-c46c-79b3-b590-800b9bd2d958" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_019e41e0-c46c-70cc-8df1-a782d451e634" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019e41e0-c46c-79b3-b590-800b9bd2d958" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e41e0-c46c-79b3-b590-800b9bd2d958_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e41e0-c46c-79b3-b590-800b9bd2d958" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e41e0-c46c-79b3-b590-800b9bd2d958_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e41e0-c46c-7432-9822-105f04f90f53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e41e0-c46c-79b3-b590-800b9bd2d958" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e41e0-c46c-7432-9822-105f04f90f53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_SalesChannelRiskMember_019e41e0-c46c-7e34-9561-c61205d03aaa" xlink:href="tndm-20260630.xsd#tndm_SalesChannelRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019e41e0-c46c-7432-9822-105f04f90f53" xlink:to="loc_tndm_SalesChannelRiskMember_019e41e0-c46c-7e34-9561-c61205d03aaa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019e41e0-c46c-762b-a4e9-74db6c2f5762" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019e41e0-c46c-7432-9822-105f04f90f53" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019e41e0-c46c-762b-a4e9-74db6c2f5762" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e41e0-c46c-73c3-8ec1-143d5a444801" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_019e41e0-c46c-70cc-8df1-a782d451e634" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e41e0-c46c-73c3-8ec1-143d5a444801" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e41e0-c46c-73c3-8ec1-143d5a444801_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e41e0-c46c-73c3-8ec1-143d5a444801" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e41e0-c46c-73c3-8ec1-143d5a444801_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e41e0-c46c-7188-a5c8-b756353d28bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019e41e0-c46c-73c3-8ec1-143d5a444801" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e41e0-c46c-7188-a5c8-b756353d28bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_SalesChannelPharmacyMember_019e41e0-c46c-7b0b-8c65-ed399b9fc63d" xlink:href="tndm-20260630.xsd#tndm_SalesChannelPharmacyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019e41e0-c46c-7188-a5c8-b756353d28bb" xlink:to="loc_tndm_SalesChannelPharmacyMember_019e41e0-c46c-7b0b-8c65-ed399b9fc63d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e41e0-c46c-79ef-a571-4d740d790e25" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_019e41e0-c46c-70cc-8df1-a782d451e634" xlink:to="loc_srt_ProductOrServiceAxis_019e41e0-c46c-79ef-a571-4d740d790e25" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e41e0-c46c-79ef-a571-4d740d790e25_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e41e0-c46c-79ef-a571-4d740d790e25" xlink:to="loc_srt_ProductsAndServicesDomain_019e41e0-c46c-79ef-a571-4d740d790e25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e41e0-c46c-703d-b778-6953e8bfebb8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e41e0-c46c-79ef-a571-4d740d790e25" xlink:to="loc_srt_ProductsAndServicesDomain_019e41e0-c46c-703d-b778-6953e8bfebb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_PumpMember_019e41e0-c46c-7558-a4b3-81aadbf49a02" xlink:href="tndm-20260630.xsd#tndm_PumpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e41e0-c46c-703d-b778-6953e8bfebb8" xlink:to="loc_tndm_PumpMember_019e41e0-c46c-7558-a4b3-81aadbf49a02" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_SuppliesAndOtherMember_019e41e0-c46c-7e3c-9b2b-5ee63acbbb5d" xlink:href="tndm-20260630.xsd#tndm_SuppliesAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e41e0-c46c-703d-b778-6953e8bfebb8" xlink:to="loc_tndm_SuppliesAndOtherMember_019e41e0-c46c-7e3c-9b2b-5ee63acbbb5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019e41e0-c46c-7444-85c9-663f61969e7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_019e41e0-c46c-7652-a110-6a18492d5f58" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019e41e0-c46c-7444-85c9-663f61969e7d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019e41e0-c46c-7e48-91c9-c482aad2f4c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_019e41e0-c46c-7652-a110-6a18492d5f58" xlink:to="loc_us-gaap_NumberOfReportableSegments_019e41e0-c46c-7e48-91c9-c482aad2f4c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_NumberOfGeographicalMarkets_019e41e0-c46c-7b8a-94a8-c6d2fe5c743f" xlink:href="tndm-20260630.xsd#tndm_NumberOfGeographicalMarkets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_019e41e0-c46c-7652-a110-6a18492d5f58" xlink:to="loc_tndm_NumberOfGeographicalMarkets_019e41e0-c46c-7b8a-94a8-c6d2fe5c743f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019e41e0-c46c-7c4a-b16b-34a88a4d18b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_019e41e0-c46c-7652-a110-6a18492d5f58" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019e41e0-c46c-7c4a-b16b-34a88a4d18b6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e41e0-c46c-70e8-a0d2-c5374194072e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_019e41e0-c46c-7652-a110-6a18492d5f58" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e41e0-c46c-70e8-a0d2-c5374194072e" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/BusinessSegmentandGeographicInformationScheduleofGeographicalMarketsandProductsDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#BusinessSegmentandGeographicInformationScheduleofGeographicalMarketsandProductsDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/BusinessSegmentandGeographicInformationScheduleofGeographicalMarketsandProductsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e41e0-c46c-7545-ba64-1276696c4295" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e41e0-c46c-7c0a-80a0-221a7f7362cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e41e0-c46c-7545-ba64-1276696c4295" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e41e0-c46c-7c0a-80a0-221a7f7362cf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e41e0-c46c-7d99-a96b-f22cea8b496a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e41e0-c46c-7c0a-80a0-221a7f7362cf" xlink:to="loc_srt_StatementGeographicalAxis_019e41e0-c46c-7d99-a96b-f22cea8b496a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7d99-a96b-f22cea8b496a_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e41e0-c46c-7d99-a96b-f22cea8b496a" xlink:to="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7d99-a96b-f22cea8b496a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7ba6-b80f-f8e7385c6eb7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e41e0-c46c-7d99-a96b-f22cea8b496a" xlink:to="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7ba6-b80f-f8e7385c6eb7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e41e0-c46c-774d-9d19-0e3a9f0fd2d9" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7ba6-b80f-f8e7385c6eb7" xlink:to="loc_country_US_019e41e0-c46c-774d-9d19-0e3a9f0fd2d9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_019e41e0-c46c-764e-b19b-be8596e6eca7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e41e0-c46c-7ba6-b80f-f8e7385c6eb7" xlink:to="loc_us-gaap_NonUsMember_019e41e0-c46c-764e-b19b-be8596e6eca7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019fa469-f9db-763a-ac4a-b8b0584c1502" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e41e0-c46c-7c0a-80a0-221a7f7362cf" xlink:to="loc_srt_ProductOrServiceAxis_019fa469-f9db-763a-ac4a-b8b0584c1502" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019fa469-f9db-763a-ac4a-b8b0584c1502_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019fa469-f9db-763a-ac4a-b8b0584c1502" xlink:to="loc_srt_ProductsAndServicesDomain_019fa469-f9db-763a-ac4a-b8b0584c1502_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019fa469-f9db-7957-9787-b752d26e6a38" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019fa469-f9db-763a-ac4a-b8b0584c1502" xlink:to="loc_srt_ProductsAndServicesDomain_019fa469-f9db-7957-9787-b752d26e6a38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_PumpMember_019fa469-f9db-7613-b9af-16f8260b4608" xlink:href="tndm-20260630.xsd#tndm_PumpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019fa469-f9db-7957-9787-b752d26e6a38" xlink:to="loc_tndm_PumpMember_019fa469-f9db-7613-b9af-16f8260b4608" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_SuppliesAndOtherMember_019fa469-f9db-786d-ae97-24270a4d903e" xlink:href="tndm-20260630.xsd#tndm_SuppliesAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019fa469-f9db-7957-9787-b752d26e6a38" xlink:to="loc_tndm_SuppliesAndOtherMember_019fa469-f9db-786d-ae97-24270a4d903e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e41e0-c46c-76b2-a1ee-184cf4c8f0ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e41e0-c46c-7545-ba64-1276696c4295" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e41e0-c46c-76b2-a1ee-184cf4c8f0ff" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.tandemdiabetes.com/role/AcquisitionsDetails" xlink:type="simple" xlink:href="tndm-20260630.xsd#AcquisitionsDetails"/>
  <link:definitionLink xlink:role="http://www.tandemdiabetes.com/role/AcquisitionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019e41e0-c46c-7d12-890d-382b125cdaaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e41e0-c46c-7730-b2f3-7927c6726b6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e41e0-c46c-7d12-890d-382b125cdaaa" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e41e0-c46c-7730-b2f3-7927c6726b6d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionAxis_019e41e0-c46c-75ad-a9d7-f0b3e792091a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e41e0-c46c-7730-b2f3-7927c6726b6d" xlink:to="loc_us-gaap_AssetAcquisitionAxis_019e41e0-c46c-75ad-a9d7-f0b3e792091a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_019e41e0-c46c-75ad-a9d7-f0b3e792091a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AssetAcquisitionAxis_019e41e0-c46c-75ad-a9d7-f0b3e792091a" xlink:to="loc_us-gaap_AssetAcquisitionDomain_019e41e0-c46c-75ad-a9d7-f0b3e792091a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_019e41e0-c46c-7188-9dc7-4a38a7091d61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AssetAcquisitionAxis_019e41e0-c46c-75ad-a9d7-f0b3e792091a" xlink:to="loc_us-gaap_AssetAcquisitionDomain_019e41e0-c46c-7188-9dc7-4a38a7091d61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_AMFMedicalAcquisitionMember_019e41e0-c46c-7825-ab92-7d8f4f04ba9f" xlink:href="tndm-20260630.xsd#tndm_AMFMedicalAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionDomain_019e41e0-c46c-7188-9dc7-4a38a7091d61" xlink:to="loc_tndm_AMFMedicalAcquisitionMember_019e41e0-c46c-7825-ab92-7d8f4f04ba9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_019e41e0-c46c-7125-a0a9-acf338afb3a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e41e0-c46c-7d12-890d-382b125cdaaa" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_019e41e0-c46c-7125-a0a9-acf338afb3a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionPriceOfAcquisitionExpected_019e41e0-c46c-7d86-8b4b-1f016a8e9d39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetAcquisitionPriceOfAcquisitionExpected"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e41e0-c46c-7d12-890d-382b125cdaaa" xlink:to="loc_us-gaap_AssetAcquisitionPriceOfAcquisitionExpected_019e41e0-c46c-7d86-8b4b-1f016a8e9d39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentAssetAcquiredOtherThanThroughBusinessCombinationWrittenOff_019e41e0-c46c-791a-877f-5e804fb7b8e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentAssetAcquiredOtherThanThroughBusinessCombinationWrittenOff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e41e0-c46c-7d12-890d-382b125cdaaa" xlink:to="loc_us-gaap_ResearchAndDevelopmentAssetAcquiredOtherThanThroughBusinessCombinationWrittenOff_019e41e0-c46c-791a-877f-5e804fb7b8e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossAfterTax_019e41e0-c46c-7b57-a8a8-6cabe6ad0dfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyTransactionGainLossAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e41e0-c46c-7d12-890d-382b125cdaaa" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossAfterTax_019e41e0-c46c-7b57-a8a8-6cabe6ad0dfd" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_tndm_ChristopherTwomeyMember_019f468b-777d-7de0-8c74-ea021a66bcdb" xlink:href="tndm-20260630.xsd#tndm_ChristopherTwomeyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_tndm_ChristopherTwomeyMember_019f468b-777d-7de0-8c74-ea021a66bcdb" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
