Background: The physical and psychosocial benefits of physical activity for typically developing youth are well established; however, its impact on youth with intellectual disabilities is not as well understood. The aims of this review and meta-analysis were to synthesize the literature and quantify the effects of physical activity on the physical and psychosocial health of youth with intellectual disabilities. Method: Studies meeting the inclusion criteria were grouped by their focus on physical health and/or psychosocial health outcomes. Meta-analyses were performed using 3-level, random effects and mixed effects models. Results: One hundred nine studies met the inclusion criteria. Physical activity had a large effect on physical health (g = 0.773, P <.001) and a moderately large effect (g = 0.682, P <.001) on psychosocial health. Participant age, intellectual disability level, other developmental disabilities, outcome type, and intervention type moderated the effects of physical activity on physical health, whereas study design, risk of bias, other developmental disabilities, outcome type, and intervention type were moderators on psychosocial health. Conclusions: Physical activity has positive effects on the physical and psychosocial health of youth with intellectual disabilities. Although resistance training shows the most physical benefits, teaching movement and sports skills appear to benefit their physical and psychosocial health.