This article examines international criminalization, the process by which particular acts come to be established as international crimes in world politics. While international legal scholars suggest international criminalization constitutes a legal process that centres on international legal codification, this article argues, by drawing upon the insights of constructivist International Relations scholarship, that it is better conceived as a social process. More specifically, the process of international criminalization involves the development of an international social consensus on international criminality, which takes hold in international society following diplomatic negotiations between social actors. Furthermore, international criminalization embraces a two-stage process that requires, firstly, the emergence of an international criminal norm and secondly, the translation of that norm into an international legal proscription. Using these conceptual insights, the article analyses, through a close analysis of international archival documents, the historical emergence of genocide, in order to demonstrate how its proposed conceptualization of international criminalization can better explain how and why this act was specifically established as an international crime. In doing so, the article offers an alternative account of genocide's criminalization which, unlike the existing literature, goes some way towards uncovering the processes of social construction that informed its establishment as an international crime.