The rapid growth of mobile technology and the increasing prevalence of mental health issues highlight the need to investigate the potential of mobile apps for promoting mental health. In this study, we conducted a comprehensive meta‐analysis to systematically review and synthesize research studies published between 2002 and 2021. The search yielded 46 eligible studies (26 comparative studies and 20 within‐group studies) evaluating the effectiveness of mental health apps (MHAs) in treating depression, with outcome data from 4207 participants. Overall, analyses showed small (for comparative studies) and medium (for within‐group studies) effect sizes favoring MHAs reducing depression symptoms. In within‐group studies, participants had significantly more treatment gain with MHA interventions lasting over 4 weeks, with over 8‐week intervals associated with the greatest reductions in depression symptoms. Gender, ethnicity, intervention type, and app type are not significant moderators.