In emerging and transitioning democracies, a new constitution is often approved by referendum. In a constitutional interregnum, where the previous constitutional system has been abrogated and there are no mechanisms of political expression in place, approval at referendum appears necessary for a new constitution to make a claim to the authority of popular sovereignty. At the same time, the new constitution’s approval at referendum appears sufficient for that claim. This paper challenges both of these views, arguing that a referendum is neither necessary nor sufficient for a constitution’s claim to the authority of popular sovereignty. By drawing a distinction between constituent power and popular sovereignty in the first place, and between popular sovereignty and sociological legitimacy in the second place, the paper argues that the claim to popular sovereignty brings with it a substantive commitment to every individual’s moral autonomy and political equality.