The present study was a comprehensive, quantitative synthesis of the literature examining the effects of physical activity on children's mental health outcomes. The final analysis included 73 published and unpublished studies, totaling 246 effect sizes. Various study and participant characteristics were coded to assess moderator effects, including type of physical activity, mental health outcome, gender, cognitive ability, mental status, and implementer of the physical activity, etc. Results demonstrated varying effects depending on the methodology of the examined study [i.e., correlational vs. randomized controlled trial (RCT)/non-RCT] and characteristics of the participants, although overall effects of physical activity on children's mental health were small but significant, indicating that on average physical activity led to improved mental health outcomes for all children.