Introduction: The high prevalence of mental health problems in prisons has become a global problem. The purpose of this study was to compare the effectiveness of group therapy based on acceptance and commitment, cognitive-behavioral therapy, and reality therapy on depression, anxiety, aggression, and physical complaints of male inmates of Isfahan Central Prison, Iran. Methods: The present study was a one-blind randomized controlled trial study (pre-test, post-test and follow-up) with a control group. The statistical population included all male prisoners of Isfahan Central Prison, from whom 120 individuals were selected based on systematic random sampling and randomly assigned to three experimental groups and one control group (per group n=30). The experimental groups received group therapy based on acceptance and commitment, cognitive-behavioral therapy, and reality therapy for 12 sessions of 90 minutes, and the control group was placed on the waiting list. Subjects completed the checklist of symptoms of mental disorders in three stages. Analysis of variance with repeated measurements and SPSS software was used to analyze the data. Results: The results of the analysis of variance with repeated measurements showed that group therapy based on acceptance and commitment therapy, cognitive-behavioral therapy, and reality therapy were effective in improving the symptoms of depression, anxiety, aggression, and physical complaints (P<0. 05). Also, according to the results of the posthoc test, there was a significant difference between the three treatment groups in the post-test in the research variables (P<0. 05). Conclusion: Group therapy based on acceptance and commitment, cognitive-behavioral therapy and reality therapy improved the symptoms of depression, anxiety, aggression and physical complaints in male prisoners, and cognitive-behavioral therapy was superior.