While the emerging debate about the disintegration of the European Union focuses on descriptive and explanatory questions, this article approaches the phenomenon from the perspective of democratic theory. Building on a concept of disintegration as a form of constitutional politics that includes various possibilities of dismantling supranational polities, I argue that disintegration gives rise to a democratic puzzle. While it must be possible, for democratic reasons, to partially or entirely reverse European integration, any such step threatens the European Union’s democratic achievements. Disintegration seems to be caught between legitimate change and regression. To address this democratic puzzle, I examine to what extent European integration has produced democracy-related ‘ratchet effects’ that limit the scope for legitimate reversal. This analysis leads to three principles of legitimate disintegration that can be applied to any supranational polity and have important implications for the post-Brexit relations between the United Kingdom and the European Union.