This article explores cross-sector partnerships as a source of business legitimacy in a time where the United Nations Sustainable Development Goals (UN SDGs) arguably are a leading agenda for social, economic and environmental development. In the SDGs and their accompanying “2030 Agenda for Sustainable Development”, partnerships are emphasized as an essential tool for implementing sustainable development, and more and more business firms are consequently engaging in partnerships with government and civil society actors to contribute to the SDGs. In the research literature, partnerships have mainly been studied separately from their institutional context. However, it is the overall argument of this article that the institutional context of partnerships has played an important role in shaping the idea of the legitimate partnership today. This article incorporates the institutional context of partnerships in the assessment of their legitimacy by looking at three aspects: partnerships as a form of extended stakeholder management, partnerships as a means for Corporate Social Responsibility (CSR) implementation in the globalized and Anthropocene era, and partnerships as a result of institutionalization and legitimation in the UN system. Finally, the article presents some current challenges to the legitimacy of partnerships and provides some concluding remarks on how these challenges should be addressed.