This article evaluates the 2019 street protests in Hong Kong following the proposal of the Fugitive Offenders and Mutual Legal Assistance in Criminal Matters Legislation (Amendment) Bill 2019, in light of the constitutional settlement of the region. Firstly, it examines the ‘constitutional morality’ of Hong Kong, that is, the moral principles underlying its foundational claims to moral authority. Secondly it analyses whether the Administration’s ‘legitimacy claims’ – its rational-normative arguments for obedience to law – follow from these constitutional moral principles. Concluding that the legitimacy claims of the Administration pursuant to the Bill proved morally unintelligible, this research finds that protest action by citizens was a logical and rational response to a perceived legitimacy claim failure. It suggests that similar protests are likely to occur for the foreseeable future given the instability of the region’s constitutional morality.