Based on self-determination theory, the present study developed and evaluated the utility a school-based intervention to change pupils' physical activity intentions and self-reported leisure-time physical activity behaviour. The study evaluated utility of the intervention to promote physical activity participation over a 5-week interval of time. A cluster randomised design targeting 215 pupils from 10 schools with schools as the unit of randomisation was adopted (Male = 106, Female = 109, Age = 14.84, SD = 0.48). Results indicated that pupils who were taught by autonomy-supportive teachers reported stronger intentions to exercise during leisure time and participated more frequently in leisure-time physical activities than pupils in the control condition. Autonomous motivation and intentions mediated the effects of the intervention on self-reported physical activity behaviour. It is concluded that self-determination theory provides a useful framework for the development of school-based interventions that ultimately affect leisure-time physical activity participation.