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Background: An increasing number of studies have investigated the efficacy of Acceptance and Commitment Therapy (ACT) for the reduction of dysregulated eating behaviours such as binge eating and emotional eating. However, little is known about their short- and long-term efficacy and underlying mechanisms of change., Objectives: To conduct a systematic effect size analysis to estimate the efficacy of ACT-based treatments on measures of dysregulated eating and of psychological flexibility, a theorized ACT mechanism of change., Methods: Literature searches were conducted in PsycInfo, Medline, Web of Science, and ProQuest Dissertations. Within-group and between-group standardized mean differences were computed using Comprehensive Meta-Analysis Version 3. Additional subgroup and meta-regression analyses by study characteristics were conducted., Results: A total of 20 publications (22 samples, n = 1269) were included. Pre-post and pre-follow-up effects suggest that ACT-based treatments are moderately effective in reducing dysregulated eating behaviours and increasing psychological flexibility. These effects were comparable for binge-eating and emotional eating outcomes and for face-to-face interventions, Web-based interventions, and interventions that used a self-help book. Longer treatments were associated with larger outcome effect sizes, and changes in psychological flexibility were not associated with changes in dysregulated eating outcomes. Small significant effects were found in favour of ACT when compared to inactive control groups. The only three studies that included active control groups and did not show significant differences in outcomes between ACT and other treatments., Conclusion: Future studies should aim to compare ACT-based treatments to active treatments and to provide empirical evidence for the theoretical mediating role of psychological flexibility in reported changes in eating behaviour., (Copyright © 2022 Elsevier Ltd. All rights reserved.)