How does regime complexity shape inter-institutional cooperation? I argue that the level of hierarchy and differentiation that emerges among the institutions within regime complexes shape the level of cooperation among them. In addition, hierarchy and differentiation in regime complexes also impact the specific patterns of cooperative relations that emerge among their components. I explore the explanatory power of my argument using the regime complex for the global governance of private security as a case study. Empirically, I draw on a unique dataset of institutional documents from the public websites of the institutions in the global private security regime complex and other originally collected data. I trace and explain the development of hierarchy and differentiation within the complex between 2000 and 2020. To empirically test my theoretical argument about the effect of hierarchy and differentiation on cooperation within regime complexes, I use machine learning methods, social network analysis, and statistical analysis. I find that both differentiation and hierarchy have shaped the patterns of cooperative relations among the institutions in the regime complex throughout its lifetime. The paper contributes to scholarship on regime complexity, global governance, and private security governance.