How do governing actors in international politics become legitimised? Current approaches to the study of legitimation do not fully account for the complexities of governance in contemporary international and global politics because they pre-specify 'sources' of legitimacy and treat change in audience expectations towards rightful rule as exogenous to legitimation processes. Instead, this article synthesises existing models of legitimation with relational theory to argue that constellations of institutional complexities necessitate an analytical focus on audiences and their expectations as embedded in governance networks. It then provides a relational theory of legitimation, emphasising the mechanisms undergirding legitimation: legitimation should be conceptualised as a process of congruence-finding between actors' normative expectations. A governance relation might be influenced towards greater or lesser congruence via several mechanisms working at the level of the relation and the wider network, with more congruence giving rise to stabler governance practices. In this way, the theory builds upon legitimation scholarship by developing pathways to investigate legitimation across the varied contexts of international politics: it avoids a normative background theory of legitimacy sources and provides an improved framework for understanding change in the legitimacy of institutions over time by considering endogenous mechanisms of legitimation.