The article conceives international (or global) constitutionalism as a legal argument which recommends and strengthens efforts (legal and political) to compensate for ongoing de-constitutionalization on the domestic level. Although the notions ‘international constitution’ and ‘international constitutionalism’ have in recent years served as buzzwords in various discourses, the many meanings of those concepts have not yet been fully explored and disentangled. This paper suggests a specific understanding of those concepts. It highlights various aspects and elements of micro- and macro-constitutionalization in international law, and identifies anti-constitutionalist trends. On this basis, the paper finds that, although no international constitution in a formal sense exists, fundamental norms in the international legal order do fulfil constitutional functions. Because those norms can reasonably be qualified as having a constitutional quality, they may not be summarily discarded in the event of a conflict with domestic constitutional law. Because the relevant norms form a transnational constitutional network, and cannot be aligned in an abstract hierarchy, conflict resolution requires a balancing of interests in concrete cases. Finally, because constitutionalism historically and prescriptively means asking for a legitimate constitution, a constitutionalist reading of the international legal order provokes the question of its legitimacy. This question is pressing, because state sovereignty and consent are – on good grounds – no longer accepted as the sole source of legitimacy of international law. International constitutionalism – as understood in this paper – does not ask for state-like forms of legitimacy of a world government, but stimulates the search for new mechanisms to strengthen the legitimacy of global governance.