The United Nations Secretariat is one of the six principal organs of the United Nations (UN), The secretariat is the UN's executive arm. The secretariat has an important role in setting the agenda for the deliberative and decision-making bodies of the UN (i.e., the General Assembly, Economic and Social Council, and Security Council), and the implementation of the decision of these bodies. The secretary-general, who is appointed by the General Assembly, is the head of the secretariat.

The mandate of the secretariat is a wide one. Dag Hammarskjöld, the UN's second secretary-general, described its power as follows: "The United Nations is what member nations made it, but within the limits set by government action and government cooperation, much depends on what the secretariat makes it. It has creative capacity. It can introduce new ideas. It can, in proper forms, take initiatives. It can put before member governments findings which will influence their actions". The United Nations Department of Political Affairs, which has a role analogous to a ministry of foreign affairs, is a part of the secretariat. So is the Department of Peace Operations. The secretariat is the main source of economic and political analysis for the General Assembly and Security Council; it administers operations initiated by UN's deliberative organs, operates political missions, prepares assessments that precede peacekeeping operations, appoints the heads of peacekeeping operations, conducts surveys and research, communicates with non-state actors such as media and non-government organizations, and is responsible for publishing all of the treaties and international agreements.

The UN secretary-general's duties include helping resolve international disputes, administering peacekeeping operations, organizing international conferences, gathering information on the implementation of Security Council decisions, and consulting with member governments regarding various initiatives. Key secretariat offices in this area include the Office of the Coordinator of Humanitarian Affairs and the Department of Peacekeeping Operations. The secretary-general may bring to the attention of the Security Council any matter that, in his or her opinion, may threaten international peace and security. The current secretary-general of the UN is António Guterres.

The secretariat is divided into offices and departments. The hierarchy within each is as follows:

As at 31 December 2022, there are 36,791 staff in the UN Secretariat from more than 190 nationalities and working across 474 duty stations. Eligibility for employment is based on a competitive application process, and there are opportunities across a variety of positions, from internships to mid-level roles and senior leadership appointments. Qualifications for membership include "the highest standards of efficiency, competence, and integrity", according to the UN Charter. Staff-members are appointed by the secretary-general alone and are assigned to the organs of the UN. Staff members are appointed on a temporary or permanent basis, under the discretion of the secretary-general. During staff recruitment, geographical representation is an especially prominent selection factor in order for the UN workforce to accurately reflect the scope of member states present in the UN. The charter states that staff members are responsible "only to the organization" and are prohibited from any action or influence that would suggest affiliation with a government or organization outside the UN.

Headquartered in New York, the secretariat functions through duty stations in Addis Ababa, Bangkok, Beirut, Geneva, Nairobi, Santiago and Vienna, in addition to offices all over the world.

One study finds the following factors play a role in the selection of staff for the secretariat: a desire to achieve a minimum number of officials from each state; population size; and lo assessment of dues. The most overrepresented states in the secretariat are small, rich democracies. The Nordic states stand out, in particular, when it comes to overrepresentation.

Representation of women in the UN, particularly at managerial and decision-making positions at the D-1 level and above, has been a UN General Assembly concern and goal since 1970. Since 1984, the UN Secretariat, in order to achieve early gender equality, issued several five-year "action plans", including strategic plans, to improve the status of women in the secretariat. These plans, however, did not have the desired impact, and progress in achieving gender parity remained slow.