This article introduces a new dataset on the access of transnational actors (TNAs) to international organizations (IOs). While IOs were long the exclusive preserve of member governments, the past decades have witnessed a shift toward more inclusive forms of governance, involving participation by non-governmental organizations, philanthropic foundations, multinational corporations, and other forms of TNAs. Yet existing research has lacked the data necessary to map this phenomenon and its variation over dimensions such as time, issue areas, and world regions. The TRANSACCESS dataset is designed for this purpose, and contains information on the level of openness in 298 bodies of 50 IOs from 1950 to 2010. On the basis of this dataset, we also introduce a first quantitative measure of institutional openness, in the shape of a composite index, available at both IO and body level. This index can be used to compare TNA access across and within IOs, but also as a variable in large-N studies on global and regional governance where IO openness is potentially relevant.