OBJECTIVE: Cancer is a leading cause of death worldwide. Religiosity is a factor that may help cancer patients to cope with their disease. The aim of the current study was to validate a Persian translation of the Muslim Religiosity Scale (MRS) in a population of Iranian patients with cancer. METHOD: Two thousand patients were invited to participate in this multisite study, of whom 1,879 participated. Patients completed a demographic questionnaire, the MRS, and several scales, including the Patient Health Questionnaire-9, the Hospital Anxiety and Depression Scale, the Perceived Social Support Scale, and the SF-12 quality of life measure. Backward-forward translation was employed to develop a Persian-language version of the MRS. Cronbach's alpha and two-week test-retest reliability were also assessed. Convergent and discriminative validity as well as the factor structure of the scale were also examined. RESULTS: The internal reliability (α) of the religious practices and beliefs subscales was 0.88 and 0.92, respectively. The intraclass correlation coefficient (ICC) was 0.92 (range = 0.75-1.0). The scale demonstrated solid convergent and discriminative validity. Factor analysis indicated two main factors, as predicted, with an appropriate goodness of fit (χ2 = 76.23, RMSEA = 0.065). Such factors as marital status, quality of life, social support, and self-efficacy were positively associated with MRS total score, while anxiety, depression, and suicide ideation had negative associations. SIGNIFICANCE OF RESULTS: The MRS is a useful tool for assessing religiosity in Iranian patients with cancer and is associated with a number of important health outcomes.