Background: This study aimed to apply the COM-B model to understand the capabilities, opportunities, and motivations for walking behaviour among adults with intellectual disabilities. Methods: A qualitative study was conducted with adults (= 18 years) with mild to moderate intellectual disabilities living in Greater Glasgow using one-to-one interviews (n = 12; women = 5) and a photo-elicitation activity followed by a focus group discussion (n = 5; women = 1). The framework approach to analysis allowed for the influences of walking to be mapped onto the COM-B model. Results: Walking is a complex behaviour with many capabilities, opportunities and motivations to consider. Adults with intellectual disabilities were involved in making decisions about what results should be prioritised. Conclusions: The COM-B model is a flexible framework that can be applied to understand health behaviours of adults with intellectual disabilities. It is imperative to work with adults with intellectual disabilities throughout the research process.