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Background: Effective antibullying interventions may reduce the impact of bullying on young people's mental health. Nevertheless, little is known about their effectiveness in reducing internalizing symptoms such as anxiety or depression, and what factors may influence intervention effects. The aim of this systematic review, meta-analysis, and metaregression is to assess the effects of school-based antibullying interventions on children's and adolescent's internalizing symptoms. The secondary aims are to explore potential moderators, intervention components, and reductions in bullying as mediators of intervention effects on internalizing symptoms., Methods: We searched nine databases: PsycINFO, Web of Science, ERIC, SCOPUS, CINAHL, Medline, Embase, ProQuest, and Cochrane Library, and performed an author search of included studies in English from January 1983 to April 2021. We included studies that evaluated school-based antibullying interventions using controlled designs and reporting on both bullying and internalizing outcomes. Random-effects and metaregression models were used to derive Hedges g values with pooled 95% CIs as estimates of effect size and to test associations between moderator variables and effect size estimates. Path analysis was used to test potential mediation using effect size measures of victimization, perpetration, and internalizing outcomes. Quality and risk of bias were assessed using Cochrane collaboration tools., Results: This review included 22 studies with 58,091 participants in the meta-analysis. Antibullying interventions had a very small effect in reducing overall internalizing symptoms (ES, 0.06; 95% CI, 0.0284 to 0.1005), anxiety (ES, 0.08; 95% CI, 0.011 to 0.158), and depression (ES, 0.06; 95% CI, 0.014 to 0.107) at postintervention. The reduction in internalizing symptoms did not vary significantly across geographic location, grade level, program duration, and intensity. The intervention component 'working with peers' was associated with a significant reduction, and 'using CBT techniques' was associated with a significant increase in internalizing outcomes. Bullying victimization and perpetration did not mediate the relationship between intervention condition and internalizing outcomes., Conclusions: Antibullying interventions have a small impact on reducing internalizing symptoms. Ongoing development of antibullying interventions should address how best to maximize their impact on internalizing symptoms to safeguard young people from the damaging mental health outcomes of bullying., (© 2022 The Authors. Journal of Child Psychology and Psychiatry published by John Wiley & Sons Ltd on behalf of Association for Child and Adolescent Mental Health.)