In his Fourth Report on the Identification of Customary International Law (2016), Special Rapporteur Michael Wood confirmed that ‘[i]n certain cases, the practice of international organizations also contributes to the expression, or creation, of rules of customary international law.’ That the practice of international organizations can be relevant when identifying customary international law is relatively uncontroversial. The practice of states within international organizations such as the UN General Assembly, for example, may contribute to the development of custom. Yet, there is little discussion about whether and how the practice of international organizations as such may contribute to the development of customary international law. This contribution discusses the organization that is the most capable of contributing to the development of customary international law in its own right: the European Union. Using examples from the EU’s treaty practice and from the Court of Justice of the EU, it argues that the Union does not simply represent the collective will of its Member States, but also is capable of contributing to customary international law in its own right.