International social alliances—in which multinational enterprises (MNEs) and social enterprises (SEs) collaborate—are a vital source for the development and scaling up social innovations for value creation. Yet, these alliances face significant legitimacy challenges, which are more glaring in bottom-of-the-pyramid markets (BOPMs) within emerging and developing economies owing to weak and underdeveloped formal institutions. Drawing on the legitimacy, institutional, and social alliances literature, we develop a conceptual framework that explains the importance of developing social, institutional, and commercial legitimacy in international social alliances operating in BOPMs. We also explored the challenges faced by international social alliances in BOPMs and the factors that enable MNEs and SEs to build different types of legitimacy. We contribute to international business research by providing an understanding of various legitimacy building strategies enacted by international social alliances based in BOPMs for social value creation.