In many issue areas of world politics, international cooperation is governed by a dense network of overlapping institutions. Intergovernmental organizations (IGOs) are at the core of such regime complexes. Yet, we know little about how this environment of regime complexity shapes the creation and design of IGOs. We offer a theoretical argument that explains the effects of institutional overlap within regime complexes on the creation and design of new IGOs. We argue that states seek to avoid the proliferation of overlapping organizations that provide similar functions to similar sets of member states in an issue area. They do so to curb institutional redundancy and limit the transaction costs and uncertainty of cooperation. When it comes to the design of IGOs, states reduce uncertainty related to the design of new IGOs by taking cues from overlapping organizations, which results in a convergence of designs across the IGOs within a regime complex. To test the implications of our argument, we introduce an innovative quantitative measure of institutional overlap in global governance and new data on the institutional design and governance tasks of the 534 IGOs contained in the Correlates of War Project’s IGO data. Findings from two-stage dyadic regression models support our theoretical argument and point to areas of future research.