Mindfulness- and acceptance-based programmes (MABPs) in the treatment of obsessive-compulsive disorder (OCD) are increasingly gaining research interest, yet a comprehensive systematic review and meta-analysis is missing. To fill this gap, we analysed 46 trials involving 2221 patients. Two independent reviewers screened records, extracted data, assessed risk of bias, and rated overall quality of evidence. MABPs were associated with large reductions in OCD-severity in between-group analysis in randomised controlled trials (k = 33
g = -.87
CI = -1.13,-.60) and within-group pre-post analysis in all MABPs (k = 49
g = -1.72
CI = -2.00,-1.44). Depressive symptoms decreased between- and within-group with a small to moderate effect, with maintained reductions at follow-up for both OCD and depression. Moderate to large pre-post improvements were also observed in anxiety, obsessive beliefs, and quality of life. MABPs did not differ from cognitive behavioural therapy and exposure and response prevention (k = 9
g=.02
CI = -.23,.26) but were superior to medication (k = 5
g = -.77
CI = -1.44,-.11) and waitlist (k = 16
g = -1.66
CI = -2.1,-1.24). Symptom reductions were observed across world regions, but to varying degrees. When combined, increases in mindfulness and psychological flexibility predicted reductions in OCD symptoms. Outcomes were not moderated by treatment duration, samples', and therapists' characteristics. MABPs can reduce OCD-severity, but further high-quality trials with long-term follow-ups are needed to confirm results.