*We thank three anonymous reviewers for their constructive comments on earlier versions of the manuscript. We are also grateful for comments from colleagues at the Centre for Global Cooperation Research (Duisburg-Essen), Environmental Sociology Section (Örebro University), the Myxa Research Group (Berlin), and the DVPW working group on the Sociology of International Relations. This article was written as part of the project Science Role in International Environmental Governance: Climate Change, Biodiversity and Air Pollution, financed by the Swedish Research Council.

A growing number of expert organizations aim to provide knowledge for global environmental policy-making. Recently, there have also been explicit calls for stakeholder engagement at the global level to make scientific knowledge relevant and usable on the ground. The newly established Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) is one of the first international expert organizations to have systematically developed a strategy for stakeholder engagement in its own right. In this article, we analyze the emergence of this strategy. Employing the “politics of legitimation,” we examine how and for what reasons stakeholder engagement was introduced, justified, and finally endorsed, as well as its effects. The article explores the process of institutionalizing stakeholder engagement, as well as reconstructing the contestation of the operative norms (membership, tasks, and accountability) regulating the rules for this engagement. We conclude by discussing the broader importance of the findings for IPBES, as well as for international expert organizations in general.