This paper explores subsidiarity as a constitutional principle in international law. Some authors have appealed to a principle of subsidiarity in order to defend the legitimacy of several striking features of international law, such as the centrality of state consent, the leeway in assessing state compliance and weak sanctions in its absence. The article presents such defences of state-centric aspects of international law by appeals to subsidiarity, and finds them wanting. Different interpretations of subsidiarity have strikingly different institutional implications regarding the objectives of the polity, the domain and role of subunits, and the allocation of authority to apply the principle of subsidiarity itself. Five different interpretations are explored, drawn from Althusius, the US federalists, Pope Leo XIII, and others. One upshot is that the principle of subsidiarity cannot provide normative legitimacy to the state-centric aspects of current international law on its own. It stands in need of substantial interpretation. The versions of subsidiarity that match current practices of public international law are questionable. Many crucial aspects of our legal order must be reconsidered – in particular the standing and scope of state sovereignty.