Background: An increasing number of studies are investigating traditional meditation retreats. Very little, however, is known about their effectiveness. Objective: To evaluate the effectiveness of meditation retreats on improving psychological outcomes in general population. Data sources: A systematic review of studies published in journals or as dissertations in PSYCINFO, PUBMED, CINAHL or Web of Science from the first available date until October 22, 2016. Review methods: A total of 20 papers (21 studies, N=2912) were included. Results: Effect-size estimates of outcomes combined suggested that traditional meditation retreats are moderately effective in pre-post analyses (n=19; Hedge's g=0.45; 95% CI [0.35, 0.54], p<0.00001) and in analyses comparing retreats to controls (n=14; Hedge's g=0.49; 95% CI [0.36, 0.61], p<0.00001). Results were maintained at follow-up. No differences were observed between meditation styles. Results suggested large effects on measures of anxiety, depression and stress, and moderate effects on measures of emotional regulation and quality of life. As to potential mechanisms of actions, results showed large effects on measures of mindfulness and compassion, and moderate effects on measures of acceptance. In addition, changes in mindfulness levels strongly moderated clinical effect sizes. However, heterogeneity was significant among trials, probably due to differences in study designs, types and duration of the retreats and assessed outcomes, limiting therefore the implications of the results. Conclusion: Meditation retreats are moderately to largely effective in reducing depression, anxiety, stress and in ameliorating the quality of life of participants.