Popular constitutionalism rarely arises in authoritarian polities. In the absence of genuine elections and referenda, aggrieved and disenfranchised citizens are more likely to resort to extraconstitutional action to defend themselves, to which the regime may respond with decisive suppression. Systemic popular constitutionalism did emerge in Hong Kong, currently under Chinese sovereignty, however. Through coordinated mass resistance based on shared constitutional understandings, large numbers of residents have succeeded in restraining the appointed Hong Kong chief executive from deploying his full range of powers, obliged China to make concessions on electoral reform it would not have made otherwise, and enabled the Basic Law, an imposed constitution, to remain relevant. This article specifies three preconditions under which the residents of Hong Kong have, in the teeth of authoritarianism, managed to adjudge the constitutionality of the acts of the ruling elite with their feet. These preconditions, nevertheless, are idiosyncratic, and may not endure the recent mounting tensions between Hong Kong and China [Correction added on 5 June 2014, after first online publication: the phrase “to authoritarian politic” has been removed from the last statement in the abstract.].