Purpose <p>This study explored socio-demographic associations and physical activity (PA) participation of autistic adolescents reported by parents. Parents also reported the patterns and frequencies of PA participation, as there is a limited understanding of the activities they engage in Ontario, Canada. There is a need for this research as autistic adolescents are less likely to participate in PA. Studying socio-demographic characteristics and understanding the activities adolescents engage in can inform policies and practice to support participation.</p> Methods <p>A cross-sectional survey was mailed to parents of 525 autistic adolescents aged 12–19 years about PA participation for the preceding year. Parents provided data about free play and organized PA activities of adolescents (<i>n</i> = 306; 63.7% male; <i>M</i>_<i>age</i> = 15.1 years), and demographic information. Multiple linear regression was used to evaluate the relationships between PA scores, activities reported and the six demographic variables. The relationship between the sedentary scores, tallied frequency scores and number of activities was also assessed using linear regression.</p> Results <p>Overall, 85% of autistic adolescents participated in at least one of the 18 free play activities. Sex, number of siblings, and income were associated with PA participation. Sedentary behaviour was negatively associated with activity frequency and parental education level. Sedentary behaviour was positively associated with income level.</p> Conclusion <p>Ensuring PA reflects socio-demographic characteristics and local priorities is critical to support participation and well-being. Tailoring programs and policies for autistic females, siblings and parent mediated interventions are areas in need of further development to support autistic adolescents in PA.</p>