Mercosur has undergone numerous transformations, from a customs union to an organization promoting development and consulting civil society. Recently, there has been a backlash against such ‘politicization’, and an attempt to return Mercosur to its origins. At each of these phases, Mercosur was plagued by questions about its legitimacy and purpose, and its leaders reached out to different types of actors to legitimize it. This article argues that each phase of Mercosur's evolution represented the ascendancy of a constellation of social forces, which sought to shift the governance of particular issues beyond the national scope, to secure their interests. In this context, the different legitimation processes and changing actors involved in them represent different balances of forces within Mercosur. The article traces the evolution of Mercosur and its legitimation processes, identifying the social struggles behind them, and the different conceptions of legitimacy constructed and contested as part of this process.