Promotion of good mental health in young people is important. Our aim was to evaluate the consistency and magnitude of the efficacy of universal/selective interventions to promote good mental health. A systematic PRISMA/RIGHT-compliant meta-analysis (PROSPERO: CRD42018088708) search of Web of Science until 04/31/2019 identified original studies comparing the efficacy of universal/selective interventions for good mental health vs a control group, in samples with a mean age <35 years. Meta-analytical random-effects model, heterogeneity statistics, assessment of publication bias, study quality and sensitivity analyses investigated the efficacy (Hedges' g=effect size, ES) of universal/selective interventions to promote 14 good mental health outcomes defined a-priori. 276 studies were included (total participants: 159,508, 79,142 interventions and 80,366 controls), mean age=15.0 (SD=7.4); female=56.0%. There was a significant overall improvement in 10/13 good mental health outcome categories that could be meta-analysed: compared to controls, interventions significantly improved (in descending order of magnitude) mental health literacy (ES=0.685, p<0.001), emotions (ES=0.541, p<0.001), self-perceptions and values (ES=0.49, p<0.001), quality of life (ES=0.457, p=0.001), cognitive skills (ES=0.428, p<0.001), social skills (ES=0.371, p<0.001), physical health (ES=0.285, p<0.001), sexual health (ES=0.257, p=0.017), academic/occupational performance (ES=0.211, p<0.001) and attitude towards mental disorders (ES=0.177, p=0.006). Psychoeducation was the most effective intervention for promoting mental health literacy (ES=0.774, p<0.001) and cognitive skills (ES=1.153, p=0.03). Physical therapy, exercise and relaxation were more effective than psychoeducation and psychotherapy for promoting physical health (ES=0.498, p<0.001). In conclusion, several universal/selective interventions can be effective to promote good mental health in young people. Future research should consolidate and extend these findings.