BACKGROUND: The present study aimed to develop an integrated protocol by combining religious content and acceptance and commitment therapy (ACT) and evaluate its effectiveness in controlling obsessive-compulsive disorder (OCD), dysfunctional beliefs, feeling guilt, scrupulosity, and thought control among Muslims in Tehran, Iran. MATERIALS AND METHODS: An exploratory mixed-method research design was applied in this study. In the qualitative stage, the document analysis method was used to extract components related to ACT with religious content. Also, the content and face validity of the intervention were confirmed by experts. Subsequently, a semi-experimental, pretest–posttest, control-group design was performed to evaluate the effectiveness of the adapted protocol with a 3-month follow-up. In the quantitative stage, the inclusion criteria were meeting the diagnostic criteria for OCD based on the diagnostic interview of a psychiatrist, having religious purity/impurity obsessions, not receiving minimum psychological treatment for at least one month before entering the study, religious commitment; minimum age of 18 years and maximum age of 50 years; and having at least a high school diploma. The exclusion criteria from the research were as follows: age over 50 years; educational level of less than a high school diploma; having a personality disorder; receiving other treatments, inability to participate in sessions; and being introduced by a family member. The experimental and control groups participated in 25 individual treatment sessions based on the adapted protocol and 8 conventional ACT sessions. RESULTS: According to the results, the effectiveness of the religion-adapted ACT intervention on the severity of obsession and dysfunctional religious beliefs was higher in patients with OCD compared to the control group in the posttest. CONCLUSION: The present study showed that adding religious components to the ACT protocol can increase its effectiveness in reducing the severity of purity/impurity obsessions compared to the conventional ACT in the Muslim Iranian population.