Why are the public, private, and nonprofit sectors increasingly difficult to distinguish? This paper offers a new systems theory-based explanation. We argue that organizations today must respond simultaneously to demands from multiple function systems—legal, economic, political, scientific, and more. This creates internal pressure to accommodate competing expectations. As organizations adapt by integrating these demands into their structures and practices, traditional sector labels lose their descriptive value. We introduce a process model that explains this transformation in three stages: rising functional turbulence, multifunctional restructuring, and the erosion of established sectoral categories. This perspective moves beyond conventional accounts of sector blurring that focus on resource dependence and institutional logics. Instead, it shows how deeper shifts in today’s functionally differentiated society are reshaping how organizations function, how they are evaluated, and how they are classified—with far-reaching implications for governance, legitimacy, and the future of organizational identity.