Background: Self-harm is a clinically relevant and prevalent behaviour which peaks in adolescence. Given the high prevalence of self-harm, the high levels of psychiatric comorbidity, and its role as a risk factor for suicide, delivering evidence-based care is critical. Methods: We conducted a systematic review and meta-analysis of the literature on treating self-harm in adolescents (12-19 years) published in the last 20 years, identifying 25 randomised controlled trials. We calculated the effect of treatment interventions relative to active control conditions in reducing self-harm, suicidal ideation and depressive symptoms. Results: Overall, treatment interventions fared slightly better than active controls in decreasing self-harm (d = 0.13, 95% CI 0.04-0.22, p = .004), suicidal ideation (d = 0.31, 95% CI 0.12-0.50, p = .001) and depressive symptoms (d = 0.22, 95% CI 0.07-0.38, p = .006). Subgroup analysis of specific therapies revealed moderate effects of DBT-A in reducing self-harm (d = 0.51, 95% CI 0.18-0.85, p = .002) and suicidal ideation (d = 0.48, 95% CI 0.17-0.80, p = .003), as well as moderate effects of family-centred therapy in the treating suicidal ideation (d = 0.58, 95% CI 0.01-1.15, p = .049).