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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a09d8e-7865-7f9b-9072-d632d29a7413,g:1c1adc80-706c-4b50-89a8-02a7728d10a8-->
<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.summitplc.com/role/Cover" xlink:type="simple" xlink:href="smmt-20260913.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesTable_01a09d8e-7865-78a8-ad2d-e140e87f3a77" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityAddressesTable_01a09d8e-7865-78a8-ad2d-e140e87f3a77" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesAddressTypeAxis_01a09d8e-7865-7e8b-8ed8-e1c69346d594" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressesAddressTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntityAddressesTable_01a09d8e-7865-78a8-ad2d-e140e87f3a77" xlink:to="loc_dei_EntityAddressesAddressTypeAxis_01a09d8e-7865-7e8b-8ed8-e1c69346d594" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_01a09d8e-7865-7e8b-8ed8-e1c69346d594_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_01a09d8e-7865-7e8b-8ed8-e1c69346d594" xlink:to="loc_dei_AddressTypeDomain_01a09d8e-7865-7e8b-8ed8-e1c69346d594_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_01a09d8e-7865-78f8-af72-f08733b45303" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_01a09d8e-7865-7e8b-8ed8-e1c69346d594" xlink:to="loc_dei_AddressTypeDomain_01a09d8e-7865-78f8-af72-f08733b45303" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_FormerAddressMember_01a09d8e-7865-7f1c-98a2-12e70763337a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_FormerAddressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_AddressTypeDomain_01a09d8e-7865-78f8-af72-f08733b45303" xlink:to="loc_dei_FormerAddressMember_01a09d8e-7865-7f1c-98a2-12e70763337a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_01a09d8e-7865-72db-9483-be0e45e351d6" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_DocumentType_01a09d8e-7865-72db-9483-be0e45e351d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_01a09d8e-7865-7a07-aaff-6466589eb785" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_DocumentPeriodEndDate_01a09d8e-7865-7a07-aaff-6466589eb785" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_01a09d8e-7865-7d05-a403-6f322fee3f57" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityRegistrantName_01a09d8e-7865-7d05-a403-6f322fee3f57" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_01a09d8e-7865-7b21-adac-3cc148ba21c0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityIncorporationStateCountryCode_01a09d8e-7865-7b21-adac-3cc148ba21c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_01a09d8e-7865-7500-850a-4c8785086711" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityFileNumber_01a09d8e-7865-7500-850a-4c8785086711" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_01a09d8e-7865-78c8-bca9-ec89a34baf7f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityTaxIdentificationNumber_01a09d8e-7865-78c8-bca9-ec89a34baf7f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_01a09d8e-7865-753b-b6a1-b29c38e757ca" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityAddressAddressLine1_01a09d8e-7865-753b-b6a1-b29c38e757ca" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_01a09d8e-7865-7398-b6b7-19de5b8a29b1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityAddressCityOrTown_01a09d8e-7865-7398-b6b7-19de5b8a29b1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_01a09d8e-7865-78ff-a0c3-36fdfd958736" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityAddressStateOrProvince_01a09d8e-7865-78ff-a0c3-36fdfd958736" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_01a09d8e-7865-7d96-a811-b2fd975408c7" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityAddressPostalZipCode_01a09d8e-7865-7d96-a811-b2fd975408c7" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_01a09d8e-7865-72c1-a11a-97afcb94ca19" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_CityAreaCode_01a09d8e-7865-72c1-a11a-97afcb94ca19" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_01a09d8e-7865-7d8e-b9e5-46b337bd0914" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_LocalPhoneNumber_01a09d8e-7865-7d8e-b9e5-46b337bd0914" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_WrittenCommunications_01a09d8e-7865-766b-8162-2d6374c1bdf0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_WrittenCommunications"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_WrittenCommunications_01a09d8e-7865-766b-8162-2d6374c1bdf0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SolicitingMaterial_01a09d8e-7865-7083-9857-63ce02290133" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SolicitingMaterial"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_SolicitingMaterial_01a09d8e-7865-7083-9857-63ce02290133" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementTenderOffer_01a09d8e-7865-766c-9723-c3ef4ced9e72" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_PreCommencementTenderOffer_01a09d8e-7865-766c-9723-c3ef4ced9e72" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementIssuerTenderOffer_01a09d8e-7865-7350-96bf-7d445b5252da" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementIssuerTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_PreCommencementIssuerTenderOffer_01a09d8e-7865-7350-96bf-7d445b5252da" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_01a09d8e-7865-7f1d-8be0-8b16a3728321" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_Security12bTitle_01a09d8e-7865-7f1d-8be0-8b16a3728321" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_01a09d8e-7865-7296-8b66-e0cd6af0425d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_TradingSymbol_01a09d8e-7865-7296-8b66-e0cd6af0425d" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_01a09d8e-7865-7d1b-a954-e81d15f5491b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_SecurityExchangeName_01a09d8e-7865-7d1b-a954-e81d15f5491b" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_01a09d8e-7865-7fcc-8be6-212894f92c84" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityEmergingGrowthCompany_01a09d8e-7865-7fcc-8be6-212894f92c84" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_01a09d8e-7865-7901-87ae-7e7a311f2d7b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_EntityCentralIndexKey_01a09d8e-7865-7901-87ae-7e7a311f2d7b" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_01a09d8e-7865-78f6-af27-d17b8b7a4ad6" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityAddressesLineItems_01a09d8e-7865-7621-8ad5-76f1ef5665bb" xlink:to="loc_dei_AmendmentFlag_01a09d8e-7865-78f6-af27-d17b8b7a4ad6" xlink:type="arc" order="21"/>
  </link:definitionLink>
</link:linkbase>
