Objectives Mindfulness-based interventions (MBIs) for children and adolescents are the subject of increasing attention in both the scientific and lay press. Data suggest that MBIs might be effective for some child and adolescent populations, but current reviews are limited due to being out-of-date, small, or overly exclusive in terms of trial design. Methods The efficacy of MBIs for children and adolescents was examined via a meta-analysis of all published clinical trials. A total of 116 studies and 9616 subjects were included. Studies were coded according to outcome category (somatic, emotional, cognitive, mindfulness, quality of life, and behavioral) and time point, and tested for moderation by trial design, data type, study setting, age, gender, country, and treatment duration. Results The efficacy of MBIs was small to moderate (d = 0.377); somatic (d = 0.40) and emotional (d = 0.39) outcomes nonsignificantly outperformed the mean, while behavioral outcomes significantly underperformed in comparison to the mean (d = 0.28). MBIs were most efficacious at 1-month follow-up (d = 0.443), with the effect waning over time. The effect size did not differ according to any moderator tested, save for data type, in which objective (d = 0.45) and parent-report (d = 0.44) data outperformed Conclusions MBIs are modestly effective for children and adolescents regardless of condition, outcome, age, gender, or country, and do not significantly differ based on the outcome assessed or the type of data collected. The benefits likely wane over time.