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Background: The minimal effect of interventions to date on increasing young adolescent girls' physical activity (PA) may be due to inadequate understanding of the mechanisms underlying behavior change, yet sparse research testing a PA intervention has examined the capacity of theories to explain PA, particularly when using objective measures. Objectives: To examine whether constructs from the Health Promotion Model and Self-Determination Theory mediated changes in moderate-to-vigorous physical activity (MVPA) following a 17-week intervention. Methods: The study was a secondary analysis of data from a group randomized trial, including 12 intervention and 12 control schools in the Midwestern U.S. Data were collected in 2012-2016. Girls (5 th-8 th grade; N = 1519) completed surveys on perceived benefits and enjoyment of PA, PA self-efficacy, social support and motivation for PA, and barriers to PA and wore accelerometers. Results: The final path model had a good fit: χ 2 (21) = 1712.44, p < .001; GFI = 0.75; CFI = 0.32; RMSEA= 0.23; SRMR= 0.19. For MVPA change from baseline to post-intervention, enjoyment (B = 24.48, p < .001) and social support (B = 30.48, p < .001) had a positive direct effect, while the intervention had a positive indirect effect through enjoyment and social support (B = 9.13, p < .001). Enjoyment (B = −13.83, p < .001) and social support (B = −17.22, p < .001) had a negative indirect effect on MVPA change from post-intervention to follow-up.