It remains unclear whether children and adolescents in the child welfare ă system (CWS) exhibit a higher prevalence of mental disorders compared ă with the general population. The objective of this study was to perform ă a systematic review and meta-analysis to assess the prevalence of mental ă disorders in the CWS. ă All of the epidemiological surveys assessing the prevalence of mental ă disorders in children and adolescents in the CWS were included. The ă pooled prevalence was estimated with random effect models. Potential ă sources of heterogeneity were explored using meta-regression analyses. ă Eight studies provided prevalence estimates that were obtained from 3104 ă children and adolescents. Nearly 1 child or adolescent of every 2 (49%; ă 95% confidence interval (CI) 43-54) was identified as meeting criteria ă for a current mental disorder. The most common mental disorder was ă disruptive disorder (27%; 95% CI 20-34), including conduct disorder ă (20%; 95% CI 13-27) and oppositional defiant disorder (12%; 95% CI ă 10-14). The prevalence of attention-deficit/hyperactivity disorder was ă estimated to be 11% (95% CI 6-15). The prevalence estimates of anxiety ă and depressive disorders were 18% (95% CI 12-24) and 11% (95% CI ă 7-15). Posttraumatic stress disorder had the lowest prevalence (4%; ă 95% CI 2-6). ă High prevalences of mental disorders in the CWS were reported, which ă highlights the need for the provision of qualified service. The ă substantial heterogeneity of our findings is indicative of the need for ă accurate epidemiological data to effectively guide public policy.