This systematic review and meta-analysis evaluates the efficacy of parent training group interventions to treat child externalizing and/or internalizing problems. A search identified 21 randomized controlled trials of parent group interventions aimed at ameliorating child externalizing and/or internalizing problems in children aged 4–12 years. Random effects meta-analyses yielded significant pooled treatment effect size (g) estimates for child externalizing (g = −0.38) and internalizing problems (g = −0.18). Child anxiety symptoms or internalizing problems evident in children with externalizing behavior problems did not change significantly following intervention. Study quality was a statistically significant moderator of treatment response for child externalizing problems, however hours of planned parent group treatment and treatment recipient were not. Findings support the use of parent group interventions as an effective treatment for reducing externalizing problems in children aged 4–12 years. Whilst statistically significant, programs had a limited impact on internalizing symptoms, indicating a need for further investigation.