An under-secretary-general of the United Nations (USG) is a senior official within the United Nations System, normally appointed by the General Assembly on the recommendation of the secretary-general for a renewable term of four years. Under-secretary-general is the third highest rank in the United Nations, after the secretary-general and the deputy secretary-general. The rank is held by the heads of different UN entities, certain high officials of the United Nations Secretariat, and high-level envoys. The United Nations regards the rank as equal to that of a cabinet minister of a member state, and under-secretaries-general have diplomatic immunity under the UN Charter.

The United Nations System consists of the United Nations' six principal bodies, the Specialized Agencies and related organizations. The UN System includes subsidiary bodies such as the separately administered funds and programmes, research and training institutes, and other subsidiary entities. Some of these organizations predate the founding of the United Nations in 1945 and were inherited after the dissolution of the League of Nations.