Background: Gender differences in physical activity (PA) trajectories during adolescence are well documented, yet little research has examined whether the determinants of these trajectories vary by child’s gender. This study is one of few prospective examinations of gender differences in the influences of psychosocial and socioenvironmental factors on changes in objectively measured PA. Methods: Students and parents from elementary and middle schools located in 2 school districts in South Carolina were enrolled in a prospective cohort study of changes in children’s PA from elementary to middle school. Measures included children’s and/or parents’ ratings of various psychosocial and socioenvironmental factors as well as objectively measured PA, children’s anthropometric characteristics, and neighborhood factors at fifth and sixth grades. Results: Parents’ reports of children’s sport and class participation, parent-reported support for PA, and neighborhood resources for PA were protective against declines in PA for both boys and girls. The effects of 2 factors—children’s self-efficacy and parents’ leisure-time PA—on changes in PA over time were moderated by the child’s gender. Conclusions: A better understanding of these dynamics may inform the development of interventions.