This study sought to integrate perceived and built environmental and individual factors into the Theory of Planned Behavior model to better understand adolescents’ physical activity. Geographical Information Systems (GIS) software was used to measure the physical environment (walkability, access to physical activity facilities). Physical activity was assessed using the ActiGraph accelerometer and the Physical Activity Questionnaire for Adolescents. 110 youngsters aged 12 to 17 years from Auckland in New Zealand participated. Social cognitive variables were better predictors of both subjective and objective PA compared to perceived environmental and built environment factors.