Introduction Children with cerebral palsy (CP) participate less in physical activities and have increased sedentary behaviour compared with typically developing peers. Participate CP is a participation-focused therapy intervention for children with CP with demonstrated efficacy in a phase II randomised controlled trial (RCT) to increase perceived performance of physical activity participation goals. This study will test the effectiveness of Participate CP in a multisite phase III RCT. Methods and analysis One hundred children with CP, aged 8–14 years, classified Gross Motor Function Classification System levels I–IV will be randomised to either (1) receive Participate CP once/week for 1 hour for 12 weeks, or (2) waitlist control, usual care group. The waitlist group will then receive Participate CP following the 26-week retention time point. Outcomes will be assessed at baseline, 12 weeks and then 26 weeks post baseline. The primary outcomes are (1) self-reported participation goal performance on the Canadian Occupational Performance Measure at 12 weeks and (2) daily time in moderate-to-vigorous physical activity. Secondary outcomes include home and community participation frequency, involvement and environmental supportiveness, contextual barriers to participation, quality of life, intrinsic motivation for physical activities, child perception of an autonomy-supportive climate for physical activities and physical literacy at 12 and 26 weeks post study entry. Ethics and dissemination The Children’s Health Queensland Hospital and Health Service, The University of Queensland and the New Zealand Health and Disability Ethics Committees have approved this study. Findings will be disseminated in peer-reviewed journals and conference presentations. Trial registration number ACTRN12618000206224.