This review studies previous research on the role of nonprofit organizations in the co-production of welfare services. The need for the review stems from the lack of systematic literature reviews related to nonprofit organizations, co-production, and welfare services. The empirical material is delimited to peer-reviewed research published in international academic journals and analyzed in relation to levels of co-production, the role of organizations, and nonprofit organizational distinctiveness. Main findings include that research has primarily studied the role of nonprofit organizations in relation to co-production and enabling favorable conditions, and that existing research identifies several potential distinctive contributions made by nonprofit organizations to co-production. Implications include encouraging co-management of welfare services, enabling co-production in welfare services beyond public organizations, and exploring how different types of organizations contribute to co-production.