The article evaluates the relationship between the Responsibility to Protect (RtoP) principle and international law. We argue that although the principle is best understood as a political commitment to implement already existing legal commitments, the RtoP is not devoid of legal content as some of its critics claim. The principle contains two sets of legal responsibilities. The first – responsibilities owed by a state towards its own population – are well-established customary principles in international law. The second – responsibilities owed by states to populations in other states – are much less well established. We argue that although RtoP does not in itself create new legal duties, states already have international legal responsibilities that relate directly to the principle's second pillar. Moreover, we identify the emergence of nascent legal thinking which suggests that a wider set of legal duties might emerge in the future.