Interpersonal conflict about religion has been associated with anger, loneliness, and psychological distress. The current research consists of a pre-registered, randomized controlled trial (N = 447) on the effectiveness of a 4-week online intellectual humility intervention for adults experiencing distressing religious conflicts. Intellectual humility is a form of meta-cognition that involves awareness that one's knowledge is imperfect; that particular thoughts and beliefs might be inaccurate or incomplete. The intervention is secular, yet spiritually-sensitive, making it appropriate for individuals of any or no religion or spirituality. Measures were administered at baseline, immediately post-intervention, and one month later. Generalized linear mixed models with robust covariance estimation and intent-to-treat principle revealed a time by condition interaction for general intellectual humility, intellectual humility about the conflict, conflict intensity, cognitive and affective self-stigma, stress, somatization, depression, anxiety, personal growth, and forgiveness of the conflict partner. There was no time by condition interaction for self-stigmatizing behavior. Pairwise contrasts with Holm's sequential Bonferroni adjustment were conducted. The intervention and waitlist control groups did not differ from one another on any variables at baseline. Immediately post intervention, the intervention group was higher than the control group in general intellectual humility, conflict-specific intellectual humility, conflict resolution, personal growth, and forgiveness of the conflict partner. Immediately post intervention, the intervention group was lower than the control group in conflict intensity, cognitive and affective self-stigma, stress, somatization, depression, and anxiety. These differences were maintained one month later for general and conflict-specific intellectual humility, intensity of the conflict, conflict resolution, cognitive and affective self-stigma, stress, personal growth, and forgiveness of the conflict partner. This research supports the efficacy of a low-cost, convenient intervention for religious conflict—an interpersonal stressor for which specialized services are limited. The results suggest promise for promoting mental health and wellbeing in this context.