Background: There is a paucity of theory-informed physical activity research with adults with intellectual disabilities. This study aimed to address this by synthesising existing literature and applying the COM-B model to understand capabilities, opportunities and motivations. Methods: A qualitative evidence synthesis was conducted and reported in accordance with PRISMA guidelines and the ENTREQ. Three databases were systematically searched up to and including February 2022. Qualitative research relating to the physical activity of adults with intellectual disabilities were included. Thematic synthesis was conducted with themes mapped onto the COM-B model. Results: Twenty-five studies were included. Influences of physical activity were identified and mapped onto the COM-B model, which also included COM-B influences of social support provided by caregivers. Conclusions: There are many complex influences of physical activity for adults with intellectual disabilities. Researchers should consider the influences contributing to caregivers' capacity to support physical activity.