There is recent scholarship suggesting that the Responsibility to Protect (R2P) has now emerged as a master concept in relation to responding to mass atrocity crimes and that the R2P can further be seen as representative of an emerging global constitutional norm. In critical response, this article provides the first attempt to systematically investigate R2P’s relationship with global constitutionalisation as well as to explore its wider implication with regard to global constitutionalism. In doing so, the article examines existing discussions of R2P and global constitutionalism, tracks the normative evolution of R2P in order to determine its current ‘stage’ of norm diffusion, and further attempts to locate the extent to which the R2P can be perceived as also part of a process of global constitutionalisation. From this analysis the article concludes that although the R2P could be labelled as, at best, a weak emerging norm, it fails to meet the more demanding signifier of an emerging constitutional norm and that there is further evidence to suggest that the R2P might be better understood as a stalled or degenerating norm.