No published studies have investigated the correlation between religiosity, spirituality, mental health, and idiopathic inflammatory myopathy (IIM) or systemic autoimmune myopathy. Therefore, we aimed to evaluate the association between religiosity/spirituality, sociodemographic factors, and the mental health of IIM patients. This is a multicenter case–control study that included 151 patients with IIMs and 95 individuals without autoimmune diseases (controls), held between August 2022 and April 2023. This study used a semi-structured questionnaire that included sociodemographic information and the juxtaposition of the following questionnaires: the Attitudes Related to Spirituality Scale (ARES); the Duke University Religion Index (DUKE), which is composed of the organizational religious affiliation (ORA), non-organizational religious affiliation (NORA), and intrinsic religiosity (IR) domains; and the General Health Questionnaire-12 (GHQ-12). Data were analyzed using Epi Info software 7.2.5 (Centers for Disease Control and Prevention, Atlanta, GA, USA). A comparison between the mean values of the ARES, DUKE, and GHQ-12 scales was made using the Wilcoxon–Mann–Whitney and Kruskal–Wallis tests. A logistic regression test was used with the variables whose difference was statistically significant in the univariate analysis. Correlation analysis was performed using the Spearman rho coefficient. A higher prevalence of evangelicals and a lower prevalence of Catholics (p < 0.050) were seen in the IIM group compared to controls. Positive association was demonstrated between IIMs and the pardo ethnicity (OR = 2.26, 95% CI = 1.20–4.25, p = 0.011), highest ORA (OR = 2.81, 95% CI = 1.53–5.15, p < 0.001), NORA (OR = 3.99, 95% CI = 1.94–8·18, p < 0.001), IR (OR = 5.27, 95% CI = 2.32–11.97, p < 0.001), and ARES values (OR = 1.08, 95% CI = 1.04–1.13, p < 0.001). Mental health levels were compared between the groups (p > 0.999). Therefore, higher levels of religiosity and spirituality were observed in the IIM group than in the control group, but there was a similar distribution of mental health levels. The following can be cited as advantages of the present study: (i) the large sample for a rare disease with the presence of a control group; (ii) the multicenter characteristic with participation from three regions of Brazil; (iii) being the first study to map aspects of religiosity, spirituality, and mental health in IIMs.