International organizations are widely believed to undermine domestic democracy. Our analysis challenges this conventional wisdom, arguing that multilateral institutions can enhance the quality of national democratic processes, even in well-functioning democracies, in a number of important ways: by restricting the power of special interest factions, protecting individual rights, and improving the quality of democratic deliberation, while also increasing capacities to achieve important public purposes. The article discusses conflicts and complementarities between multilateralism and democracy, outlines a working conception of constitutional democracy, elaborates theoretically the ways in which multilateral institutions can enhance constitutional democracy, and discusses the empirical conditions under which multilateralism is most likely to have net democratic benefits, using contemporary examples to illustrate the analysis. The overall aim is to articulate a set of critical democratic standards appropriate for evaluating and helping to guide the reform of international institutions.