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Background and aims From residential programs to outpatient services, group therapy permeates the clinical field of substance misuse. While several group interventions for substance use disorders (SUDs) have demonstrated effectiveness, the existing evidence on group therapy has not been systematically reviewed. The current meta-analysis aims to provide estimates of the efficacy of group therapy for SUDs in adults using rigorous methods. Methods We included studies comparing group psychotherapy to no treatment control groups, individual psychotherapy, medication, self-help groups, and other active treatments applying no specific psychotherapeutic techniques for patients with substance use disorder. The primary outcome was abstinence, and the secondary outcomes were frequency of substance use and symptoms of substance use disorder, anxiety, depression, general psychopathology, and attrition. A comprehensive search was conducted in Medline, Web of Science, CENTRAL, and PsycINFO, complemented by a manual search. Random-effects meta-analyses were run separately for different types of control groups. Results Thirty-three studies were included. Significant small effects of group therapy were found on abstinence compared to no treatment, individual therapy, and other treatments. Effects on substance use frequency and SUD symptoms were not significant, but significant moderately sized effects emerged for mental state when group therapy was compared to no treatment. There were no differences in abstinence rates between group therapy and control groups. These results were robust in sensitivity analyses and there was no indication of publication bias. Conclusions The current findings represent the best available summary analysis of group therapy for SUDs in adults, however cautious interpretation is warranted given the limitations of the available data.