Background: Many individuals with intellectual disabilities are not sufficiently active for availing health benefits. Little is known about correlates of physical activity among this population on which to build health promotion interventions. Materials and Methods: We developed scales for measurement of self-efficacy and social support for leisure physical activity among adults with intellectual disabilities. Study participants were 152 adults with mild to moderate intellectual disabilities. Results: The scales demonstrated good internal consistency and fair to excellent test-re-test reliability. Confirmatory factor analyses revealed good fit for each of the scales, supporting the hypothesized structure of each. Scales also correlated with self-reported leisure physical activity, providing evidence of construct validity. Conclusions: Studies of physical activity correlates are needed to formulate evidence-based programmes to promote physical activity participation. This measure demonstrated good psychometric properties, and will be useful for examining social support and self-efficacy for physical activity by adults with intellectual disabilities.