Background: Adolescents with intellectual disabilities are insufficiently physically active. Where interventions have been developed and delivered, these have had limited effectiveness, and often lack a theoretical underpinning. Aim: Through application of the COM-B model, our aim is to explore the factors influencing adolescent physical activity within schools. Methods: A qualitative methodology, using focus groups with students who have mild/moderate intellectual disabilities, their parents'/carers' and teachers'. The COM-B model provided the lens through which the data were collected and analysed. Results: We identified of a range of individual, interpersonal, and environmental factors influencing physical activity, across all six COM-B constructs, within the context of the 'school-system'. Conclusion: This is the first study to use the COM-B model to explore school-based physical activity behaviour, for adolescents with intellectual disabilities. Identification of such physical activity behavioural determinants can support the development of effective and sustainable interventions.