Abstract: The legitimacy of international institutions has in recent years received growing interest from scholars, yet analyses of stakeholder perceptions of the legitimacy of institutions that coexist within a governance field have been few in number. Motivated by the proliferation of institutions in the field of global climate and energy governance, this study maps stakeholder perceptions of legitimate institutions and their sources of legitimacy in global renewable energy governance. Specifically, the article makes three contributions to the existing literature. Theoretically, it unpacks the legitimacy concept and offers a multidimensional conception of legitimacy. Methodologically, it captures these different dimensions of legitimacy by relying on three open survey questions. Empirically, it maps legitimacy perceptions among climate and energy experts and not only shows which institutions are considered most legitimate, but also why they are considered legitimate and how this varies between different stakeholders. The article thereby contributes to the literature on legitimacy by providing new insights into the sources of legitimacy among international institutions that operate under institutional complexity.