The declining number of U.S. volunteers is troubling, necessitating improved understanding of drivers of volunteer retention such as volunteer engagement. We utilized the job demands-resources model to investigate the moderating role of community service self-efficacy (CSSE) on the relationships between two demands (organizational constraints and role ambiguity) and volunteer engagement. Volunteers (N = 235) from three U.S. nonprofit organizations participated in a survey as part of a volunteer program assessment. Volunteers who encountered greater organizational constraints and role ambiguity were less engaged. In addition, CSSE attenuated the negative relationship between organizational constraints and engagement, but not the negative association between role ambiguity and engagement. When faced with organizational constraints, volunteers with higher CSSE reported greater engagement than those with lower CSSE. Organizations should therefore assess and support volunteers’ CSSE to bolster their engagement when faced with demands. Further recommendations for increasing volunteer engagement are discussed.