Background To investigate the effects of booster sessions in cognitive behavioral therapy (CBT) for children and adolescents with mood or anxiety disorders, whereas controlling for youth demographics (e.g., gender, age), primary diagnosis, and intervention characteristics (e.g., treatment modality, number of sessions). Methods Electronic databases were searched for CBT interventions for youth with mood and anxiety disorders. Fifty‐three (k = 53) studies investigating 1,937 youth met criteria for inclusion. Booster sessions were examined using two case‐controlled effect sizes: pre–post and pre–follow‐up (6 months) effect sizes and employing weighted least squares (WLSs) regressions. Results Meta‐analyses found pre–post studies with booster sessions had a larger effect size r = .58 (k = 15; 95% CI = 0.52–0.65; P < .01) than those without booster sessions r = .45 (k = 38; 95% CI = 0.41–0.49; P < .001). In the WLS regression analyses, controlling for demographic factors, primary diagnosis, and intervention characteristics, studies with booster sessions showed larger pre–post effect sizes than those without booster sessions (B = 0.13, P < .10). Similarly, pre–follow‐up studies with booster sessions showed a larger effect size r = .64 (k = 10; 95% CI = 0.57–0.70; P < .10) than those without booster sessions r = .48 (k = 20; 95% CI = 0.42–0.53; P < .01). Also, in the WLS regression analyses, pre–follow‐up studies showed larger effect sizes than those without booster sessions (B = 0.08, P < .01) after accounting for all control variables. Conclusions Result suggests that CBT interventions with booster sessions are more effective and the effect is more sustainable for youth managing mood or anxiety disorders than CBT interventions without booster sessions.