Abstract / Summary
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.