IMPORTANCE: Climate change is associated with increasing mental health morbidity and mortality. However, an umbrella review to classify and quantify the global evidence on climate-related and nature-based mental health interventions is lacking. OBJECTIVE: To assess associations of climate-related and nature-based mental health interventions with mental health outcomes. DATA SOURCES: PubMed, PsycINFO, Web of Science, and Cochrane databases were searched from inception to November 17, 2024. STUDY SELECTION: Systematic reviews with meta-analyses (SRMAs) with controlled climate-related or nature-based mental health interventions and mental health outcomes were included. DATA EXTRACTION AND SYNTHESIS: Standardized mean differences (SMDs; intervention vs control) and 95% CIs were synthesized, evidence was stratified according to the level of credibility, and associations were assessed using meta-regression. MAIN OUTCOMES AND MEASURES: Outcomes were mental disorders, psychiatric symptoms, and positive mental health. RESULTS: Twenty-eight SRMAs were included that examined 344 studies and 91 associations between psychosocial or nature-based interventions and outcomes. Of the 91 associations, 10 (11%) had a moderate credibility of evidence and 81 (89%) had low or very low credibility. Psychosocial interventions addressing climatic impact drivers were associated with very low credibility, based on limited data. Nature-based interventions were associated with reductions in tension (SMD, -0.87; 95% CI, -1.31 to -0.43), fatigue (SMD, -0.80; 95% CI, -1.16 to -0.44), confusion (SMD, -0.65; 95% CI, -1.12 to -0.19), and negative affect (SMD, -0.51; 95% CI, -0.85 to -0.16), as well as increases in positive affect (SMD, 0.98; 95% CI, 0.65 to 1.30), vigor (SMD, 0.83; 95% CI, 0.37 to 1.28), and well-being (SMD, 0.40; 95% CI, 0.07 to 0.73), with moderate credibility of evidence and not addressing climatic impact drivers. Older participants and study locations with lower tree cover, better health care access and quality, and lower systemic vulnerability to climate change were associated with stronger improvements in negative affect following nature-based interventions. CONCLUSIONS AND RELEVANCE: There is limited evidence for mental health interventions to reduce adverse mental health impacts of climatic impact drivers, but there is promising potential for future research in this field based on evidence from contexts other than climate change. Currently, strategies for mental health interventions in the context of climate change, such as those for implementing and scaling interventions, need to rely largely on global evidence from contexts other than climate change.