The purpose of this study was to evaluate the efficacy of group psychotherapy with incarcerated offenders. Meta-analytic techniques were applied to 26 empirical studies that used a treatment group versus a control group. The outcome measures assessed in this study included institutional adjustment, anger, anxiety, depression, interpersonal relations, locus of control, and self-esteem. The results indicate that positive treatment effects were found for the use of group psychotherapy with incarcerated offenders across all outcomes. Supplemental analyses were also included to identify factors that contribute to the efficacy of group psychotherapy and indicate that the use of homework exercises resulted in significantly improved outcomes. Furthermore, participants mandated to treatment did not negatively influence the efficacy of group psychotherapy.