Aggressive behaviour among adolescents has significant social and economic costs. Numerous attempts have been made to intervene to reduce aggression in adolescents. However, little is known about what factors enhance or diminish intervention effectiveness. The present systematic review and meta-analysis, therefore, seeks to quantify the effectiveness of interventions to reduce aggressive behaviour in adolescents and to identify when and for whom such interventions work best. Sixteen databases were searched for randomised controlled trials that assessed interventions to reduce aggression among adolescents. After screening 9795 records, 95 studies were included. A multi-level meta-analysis found a significant overall small-to-medium effect size (d = 0.28; 95% CI [0.17, 0.39]). More effective interventions were of shorter duration, were conducted in the Middle East, were targeted at adolescents with higher levels of risk, and were facilitated by intervention professionals. Potentially active ingredients were classified using the Behaviour Change Technique Taxonomy. Behavioural practice and problem solving were components of more effective interventions targeted at the general population. Overall the findings indicate that psychosocial interventions are effective in reducing adolescent aggression. Future trials need to assess the effect of individual techniques and their combination to identify the key components that can reduce aggression in adolescents.