This paper provides the first systematic investigation of the participation of intergovernmental organizations (IGOs) in transnational public-private governance initiatives (TGIs). In TGIs, IGOs cooperate with governments, business, and civil society organizations to address global or transnational problems. However, not all IGOs become involved in TGIs, and those who do participate vary in the extent of their involvement as well as the types of initiatives they join. Using the Transnational Public-Private Governance Initiatives in World Politics Data, I empirically map the variation of IGO involvement in 636 TGIs created in the period between 1885 and 2017. I develop theoretical explanations for the observed variation and empirically probe the explanatory power of these arguments using statistical analysis on a dataset of IGO-TGI dyads. I find that factors at the IGO, TGI, and dyadic level shape whether IGOs become involved in transnational governance initiatives. My findings provide an important empirical basis for advancing research on IGO orchestration, inter-organizational relations in global governance, and regime complexity.