In recent decades, one of the most prominent trends in global governance has been the increasing openness of international organizations (IOs) toward non-state actors (NSAs). As participants in the policymaking bodies of IOs, NSAs have become important actors in global governance, providing policy-relevant information to decision makers, representing constituency interests, and ultimately, influencing policies. Yet to date, we have little systematic comparative knowledge about the NSAs that participate in IO bodies. This article introduces the Non-State Participation in International Organizations (N-PINION) dataset, which maps NSA participant populations across eighty one policymaking bodies in twenty four global IOs between 1998 and 2017. The dataset provides indicators on the density, diversity, and volatility of NSA populations, available yearly across IO bodies. These indicators provide potential dependent and independent variables for scholars interested in a wide variety of research questions relating to the drivers, consequences, and normative merits of NSA participation in IOs.