Social enterprises, like almost all organizations, continuously strive for external legitimacy. To be perceived as externally legitimated by society, social enterprises often engage in strategic partnerships. However, scholars have only recently turned their attention to the legitimating function of such partnerships. The purpose of this article is to address the hitherto neglected construct of inter-partner legitimacy. Drawing on institutional theory, we hypothesize that such inter-partner legitimacy affects the resource transfer among partners, which will, in turn, be recognized by society and will subsequently affect each partner’s external legitimacy. Dyadic data of 121 strategic partnerships between social enterprises and their key partners confirm our hypotheses. We add to institutional theory by integrating the often ignored dimension of inter-partner legitimacy into our analysis and demonstrate its relevance to each partner’s external legitimacy and to resource transfer. Further contributions to the literature on institutional theory, legitimacy, social entrepreneurship, and inter-organizational relationships are identified.