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Question The rising prevalence of mental health conditions and a global treatment gap demand new solutions that address symptoms and foster psychological well-being. Just-in-time adaptive interventions (JITAIs) and ecological momentary interventions (EMIs) are emerging mobile health approaches, providing real-time, personalised support. However, the effectiveness of current JITAIs/EMIs and the longevity of effects remain uncertain.Study selection and analysis Studies investigating the effectiveness of JITAIs/EMIs for depression, anxiety and indicators of psychological well-being, published between 2018 and May 2025, were eligible. Following the standards for reporting (Preferred Reporting Items for Systematic Reviews and Meta-Analyses; PRISMA) and quality assessment (Risk of Bias; RoB), a total of K=23 studies and N=2563 individuals (71.7% female) were included.Findings JITAIs/EMIs showed a small between-group effect (g=0.15, 95% CI 0.05 to 0.26, p=0.003). Nine studies reported follow-up effects (mean follow-up time M=3.06 months, SD=2.21) with significant results at 1 and 3–6 months. Interventions shorter than 6 weeks yielded greater longevity of effects (g=0.71, 95% CI 0.18 to 1.24, p=0.008). Funnel plots and sensitivity analyses confirmed robustness of findings. Risk of bias was moderate to high for intervention adherence and missing outcome data.Conclusions Currently existing JITAIs and EMIs slightly improve mental health, particularly mental illness, with long-term effects up to 6 months. A clear definition of JITAIs and decision rules, research on long-term effects and careful selection of control conditions are needed.