The role of international organisations in the formation and expression of customary international law remains quite controversial as shown in the works of the International Law Commission leading to the adoption on second reading of the draft conclusions on identification of customary international law. As an international organisation of a universal character, the United Nations holds a unique place in this regard. The contribution of some of its organs to the formation and/or expression of rules of customary international law is complex and manifold. It may be direct, or indirect; and it bears a close link with the States’ reaction to positions taken by the relevant United Nations organs. This chapter examines the specific contribution of the United Nations Secretariat to the formation and expression of rules of customary international law. Such contribution is apprehended through various forms of conduct of the Secretariat, including legal opinions of the Secretariat, the Secretary-General’s practice relating to multilateral treaty depositary, and the interaction between the Secretariat and other United Nations organs. Based on this analysis, it is advanced that the United Nations Secretariat plays a direct and indirect role in the formation and expression of rules of customary international law.