Drawing on recent scholarship on international criminalisation, this article demonstrates how this concept is not only critical for explaining why certain global atrocities were recognised as international crimes but also why others failed to be criminalised in world politics. To do so, it focuses on piracy, an act that has been conventionally depicted as the first international crime to have been established within the international legal order but was subsequently excluded from the existing list of current international crimes. Guided by a conceptualisation of international criminalisation as a process that embraces, firstly, the emergence of an international criminal norm and, secondly, the translation of such a norm into an international legal proscription, the article analyses four historical periods across the twentieth century during which piracy was the subject of international debate amongst legal diplomats. Through a close analysis of primary documents from this period, it shows how piracy failed to be recognised as an international crime principally because an international criminal norm against piracy failed to emerge in world politics across this period.