Impacts of digital third-wave interventions for adults on wellbeing and mental health outcomes: A systematic review and meta-analysis of randomized controlled trials

To synthesise the efficacy of digital third-wave process-based interventions (e.g., ACT, Mindfulness, Compassion) for improving wellbeing and ameliorating mental health symptoms in adults. A prospectively registered systematic review (OSF: 10.17605/OSF.IO/5RMU2 ) and random-effects meta-analysis of randomised controlled trials (RCTs) was conducted across six databases through January 2026. Certainty of evidence was assessed using the GRADE framework. Interventions ( k  = 147; N  = 26,391) yielded small-to-moderate immediate post-intervention improvements in wellbeing ( k  = 46, g  = 0.36), depression ( k  = 110, g  = 0.44), anxiety ( k  = 93, g  = 0.40), stress ( k  = 66, g  = 0.46), quality of life ( k  = 32, g  = 0.30), and psychological flexibility ( k  = 51, g  = 0.41). Heterogeneity was substantial across most outcomes ( I2  > 75%). Benefits were observed across healthy (61 trials; n  = 15,770), mental health (41 trials; n  = 6,130), and medical samples (46 trials; n  = 4,491), with population type not significantly moderating immediate post-intervention effects. Comparator type significantly moderated several analyses, with smaller effects against active than inactive controls. Guidance did not significantly moderate immediate post-intervention effects, although it predicted larger short-term depression effects ( QM  = 4.17, p  = .048). At short-term follow-up, effects remained statistically supported across all outcomes, while only depression, anxiety, and stress remained statistically supported at medium-term follow-up. GRADE certainty was moderate for most immediate post-intervention outcomes and low for quality of life. Digital third-wave interventions produced small-to-moderate average benefits across symptom-focused, wellbeing-focused, and process outcomes. Effects were smaller against active comparators, indicating that pooled benefits should be interpreted in relation to comparator intensity. Future research should prioritise rigorous active controls, longer follow-up, and direct tests of whether changes in psychological flexibility mediate improvements in symptoms and wellbeing.