Background: Although a growing body of literature has explored the physical activity experiences from the perspective of children on the autism spectrum, the perspective of autistic adults remains largely unheard. Due to this absence of perspective, there exists limited knowledge of the appropriateness and generalizability of current models and theories of physical activity for this population. Methods: A constructivist grounded theory study was conducted to explore the experiences of adoption and maintenance of physical activity from the direct perspective of autistic adults. Autistic adults (n=23) from the United States and the United Kingdom were recruited. Results: A total of 29 codes emerged from the coding process. These codes were formed into four broad categories: (1) individual attributes; (2) environmental factors; (3) social relationships; and (4) social experiences. The interconnectedness of these four categories was explored. Conclusions: The findings and presented model highlight the importance of building successful experiences for young children on the autism spectrum, so that they are more likely to continue physical activity into their adult life. Furthermore, findings emphasize the importance of creating noncompetitive, sensory-friendly physical activity experiences for autistic adults that offer flexibility in social engagement.