Purpose: Grounded in the trans-contextual model of motivation framework, this cluster-randomized trial examined the effectiveness of an autonomy supportive physical education (PE) instruction on student motivation and physical activity (PA). Method: The study comprised six middle schools and 408 students (M[subscript age] = 12.29), with primary outcomes of students' autonomous motivation in PE and exercise, PA intention, and self-report PA. Results: The path analysis showed that the intervention had a weak positive effect (? = 0.11) on students' perception of autonomy (R[superscript 2] = 0.51), which in turn had a moderate positive effect (ß = 0.29) on autonomous motivation in PE (R[superscript 2] = 0.51). Students' perception of autonomy during PE had a positive effect (ß = 0.63) on their autonomous exercise motivation (R[superscript 2] =0.44) and autonomous exercise motivation had a positive effect (ß = 0.37) on PA intentions (R[superscript 2] = 0.34). Autonomous exercise motivation was an indirect link transferring PE autonomy to PA intentions and participation. Conclusion: The study provides evidence on how motivational experiences in PE transfer to out-of-school PA.