The EU's Prodi doctrine stipulates that a new state formed through secession from an EU member state will be treated as a third party vis-a-vis the Union. This article engages with debates on secessionism, self-determination, and democracy in the EU to discuss what the doctrine entails for the democratic legitimacy of the EU. We argue that the doctrine is only compatible with one source of political legitimacy that the EU partly depends on, the state consent model. However, it is not compatible with the voluntary association model which is a moral basis that is increasingly relevant for a supranational union of democratic states. The prevalent practice of organizing referendums on EU accession shows that securing popular support is today an important feature of the politics of legitimacy in the EU. We illustrate our argument with the case of Catalonia and contrast it with cases from the history of European integration.