International constitutionalism relates to processes of limiting traditionally unrestricted powers of states as ultimate subjects, law-makers and law-enforcers of international law. Human rights occupy a central, but very confusing and confused role in the theorisation of international constitutionalism. If feminist scholars have criticised the inadequacies, shortcomings and gaps of international law of human rights at least since 1991, the doctrine of international law theorising constitutionalisation of international law until now has remained blind to these critiques idealising human rights and often using them as the ultimate legitimating factor. Thus, legitimacy and legality become confused and the distinction between them blurred in the doctrine of international constitutionalism. This in turn creates a danger of failure of the constitutionalists project itself, as it will serve to reinforce existing inadequacies and gaps in human rights protection. To illustrate this argument, I discuss some examples related to the protection of women's and migrants' rights. In order to avoid this dangerous development, I argue that international lawyers theorising international constitutionalism shall adopt an adequate, inclusive notion of legitimacy. In order to develop this adequate understanding of legitimacy, they should first take seriously feminist and other critiques of international human rights law and international law more generally. In the final parts of this article I develop my own more detailed proposals on the future of legitimacy and international constitutionalism. In doing so, I draw on the 'self-correcting learning process' developed in the writings of Jürgen Habermas, 'democracy to come' and more general views on the nature of sovereignty and human rights expressed by Jacques Derrida, as well as Levinasian 'responsibility-to-and-for-the-Other'.