Objective: To examine physical activity self-efficacy as a mediator of the association between perceived barriers to PA and moderate-to-vigorous physical activity (MVPA) among alternative high school (AHS) students. Methods: Students (N = 145) from 6 AHS completed self-report questionnaires. Results: Mediation analyses revealed partial mediation of PA self-efficacy on relationships between general barriers to PA and MVPA (b = -0.39 reduced b = -0.33) among females (47.6% of sample). Conclusions: Interventions with female AHS students should include a component on building PA self-efficacy. However, results suggest the broader environment may have greater impact on MVPA than individual-level psycho-social factors.