Objective: Today, a high rate of prison organization costs constitutes the crimes relating to addiction and rehabilitation; furthermore, relapse into addiction destroys all such expenses. In the same line, the present study aimed at investigating the effectiveness of acceptance and commitment group therapy in reducing depression, stress, and anxiety among ex-addicts in Tabriz central prison. Method: For this purpose, 50 clients were selected through convenience sampling method from among the statistical population of 189 male ex-addicts imprisoned in the central prison of Tabriz and were randomly divided into two groups. The participants responded to DASS questionnaire before and after the administration of Acceptance and Commitment Group Therapy. Results: The results of this study showed that Acceptance and Commitment Group Therapy has had a positive effect on the reduction of stress and anxiety among the clients, but it has not had any significant effect on depression. Conclusion: In addition to Acceptance and Commitment Group Therapy, it is advisable to use other complementary therapies. In this regard, researchers are suggested to consider the long-term effects of this therapy, as well.