AbstractThe governance turn in political research has led to increased attention to informal institutions. For scholars of international relations this has contributed to recent scholarship that reveals a notable growth in the number of informal intergovernmental organizations (IIGOs). Many aspects of IIGOs remain unknown, including whether they involve transnational actors (TNAs). Yet, whether IIGOs are open to TNAs or not may affect their performance and legitimacy. Given the importance of TNA access to IIGOs, this article explores IIGOs openness to TNAs. We illustrate that IIGOs vary whether they are open or not and that arrangements for TNA access differ. Theoretically, we build on existing literature to posit that the political costs of involvement, TNA resources, and the institutional environment affect whether IIGO are open or closed to TNAs. Empirically, we present new data on TNA access to 94 IIGOs and examine the variation in IIGO openness to explore the validity of our theoretical expectations. We find that no single account can be offered to understand access across IIGOs, and our explanatory factors show variegated effects across different subgroups of the sample. Our findings have implications for debates on the rise of informal global governance and the openness of global governance.