People frequently engage in mate retention behaviors to prevent a romantic partner's infidelity or relationship defection. Because many religions strongly emphasize commitment and fidelity in relationships, religious beliefs may facilitate mate retention behaviors. However, previous research has not investigated how perceptions of romantic partners' religious beliefs are related to religiosity and mate retention behavior. A sample of adults (n = 680) involved in long-term, committed relationships completed an online survey assessing self-reported religiosity, perceptions of partners' religiosity, and mate retention behaviors. Results indicated that self-reported religiosity is associated with cost-inflicting and benefit-provisioning behavior, and that the association between religiosity and benefit-provisioning mate retention depended on perceptions of partners' religiosity and participant sex. Specifically, religious individuals who perceived their partners as less religious performed more benefit-provisioning behaviors, and religious women performed more cost-inflicting behaviors when they perceived their partners as highly religious. These findings corroborate and extend upon previous cross-cultural research by contextualizing the link between religiosity and mate retention.