Anxiety, depression, and attention-deficit/hyperactivity disorder (ADHD) affect up to 20% of children and adolescents. Despite demonstrated efficacy, evidence-based treatments for these conditions are often inaccessible; innovative solutions are essential to meet the demand for pediatric mental health care. To examine the efficacy and moderators of gamified DMHIs for anxiety, depression, and ADHD in randomized clinical trials (RCTs) for children and adolescents. A systematic search of PubMed, PsycInfo, and Web of Science was conducted for RCTs published before March 20, 2024. RCTs that evaluated the efficacy of gamified DMHIs for treating pediatric ADHD, depression, or anxiety were included. Studies were excluded if they did not use a gamified DMHI, provide sufficient data for effect sizes, or were unavailable in English. Efficacy data were extracted from rating scales for ADHD, depression, and anxiety. Extracted moderator variables included participant characteristics (eg, age and sex), intervention characteristics (eg, delivery modality and time limit), and trial design characteristics (eg, outcome measure and risk of bias). The primary outcome was change in ADHD, depression, or anxiety severity in the treatment group compared to the control group. Hedges g quantified treatment effects. The search strategy identified 27 RCTs that included 2911 participants across ADHD, depression, and anxiety disorders. There were modest significant effects of gamified DMHIs on ADHD (g, 0.28; 95% CI, 0.09 to 0.48) and depression (g, 0.28; 95% CI, 0.08 to 0.47) but small, nonsignificant effects for anxiety disorders (g, 0.07; 95% CI, -0.02 to 0.17). Moderator analyses revealed that DMHIs for ADHD delivered on a computer and those RCTs that had a greater preponderance of male participants produced larger treatment effects. DMHIs for depressive disorders that used preset time limits for gamified DMHIs also exhibited larger treatment effects. The findings suggest a benefit of gamified DMHIs for youth with ADHD or depressive disorder. Pediatricians and other health care professionals have new information about novel, accessible, and efficacious options for pediatric mental health care.