Objective: Externalizing behavior concerns (EBCs) are a common concern among youth and can be associated with significant problems, indicating the need for effective treatment. While research on psychosocial interventions is promising, the specific efficacy of youth group therapy has not been assessed in a meta-analysis. The aim of this study was to assess group therapy’s efficacy for EBCs and evaluate treatment moderators. Method: Major databases were used to identify articles using inclusion criteria. Included studies (a) were randomized controlled trials published in 1990 or later, (b) involved the treatment of behavioral concerns, (c) assessed the efficacy of child group treatment, (d) included at least one comparison condition, and (e) were in English. Included studies were analyzed in a random effects meta-analysis. Results: We analyzed 30 studies representing 2,818 youth (Mage = 10.9 years; 27.4% female). Group therapy showed a significant, medium effect on EBCs compared to passive controls (g = .50). There were also significant effects on aggressive cognitions, anger, internalizing symptoms, and social problems. Furthermore, effects on EBCs among studies using active comparators generally produced outcomes in favor of group. Conclusions: These findings indicate that group therapy is associated with significant improvements in EBCs and various secondary outcomes, with outcomes being comparable to what has been reported in past research on psychosocial treatments. Future research is needed to clarify discrepancies and replicate the present findings.