The United Nations Secretariat Building is a skyscraper at the headquarters of the United Nations in the Turtle Bay neighborhood of Manhattan in New York City. It contains the offices of the United Nations Secretariat, the executive organ of the United Nations (UN). The building, designed in the International Style, is 505 ft (154 m) tall with 39 above-ground stories. It was designed by a group of architects led by Wallace Harrison. Although the building is located within the United States, the site is under UN jurisdiction, so the building is exempt from some local regulations.

The Secretariat Building is designed as a rectangular slab measuring 72 by 287 ft (22 by 87 m); it is oriented from north to south and is connected with other UN headquarters buildings. The wider western and eastern elevations of the facade are glass curtain walls, while the narrower northern and southern elevations are made of marble. The Secretariat Building has 889,000 ft² (82,600 m²) of space. There are press offices, staff rooms, and other functions on the lower stories. The Secretariat offices are placed on the upper stories, which were originally arranged in a modular layout. The building also features various pieces of artwork. The building's style has inspired the construction of other glass curtain wall buildings in Manhattan.

The design process for the United Nations headquarters formally began in February 1947, and a groundbreaking ceremony for the Secretariat Building occurred on September 14, 1948. Staff started moving into the building on August 21, 1950, and it was completed in June 1951. Within a decade, the Secretariat Building was overcrowded, prompting the UN to build additional office space nearby. The building started to deteriorate in the 1980s due to a lack of funding, worsened by the fact that it did not meet modern New York City building codes. UN officials considered renovating the building by the late 1990s, but the project was deferred for several years. The Secretariat Building was renovated starting in 2010 and reopened in phases from July to December 2012.

The Secretariat Building is part of the headquarters of the United Nations in the Turtle Bay neighborhood of Manhattan in New York City. It occupies a land lot bounded by First Avenue to the west, 42nd Street to the south, the East River to the east, and 48th Street to the north. Although it is physically within the United States, the underlying land is under the jurisdiction of the United Nations (UN). The site is technically extraterritorial through a treaty agreement with the US government, though it is not a territory governed by the UN. Most local, state, and federal laws still apply within the UN headquarters. Due to the site's extraterritorial status, the headquarters buildings are not New York City designated landmarks, since such a designation falls under the purview of the New York City Landmarks Preservation Commission.

The Secretariat Building is directly connected to the Conference Building (housing the Security Council) at its northeast and the Dag Hammarskjöld Library to the south. It is indirectly connected to the United Nations General Assembly Building to the north, via the Conference Building. West of the Secretariat Building is a circular pool with a decorative fountain in its center, as well as a sculpture executed in 1964 by British artist Barbara Hepworth in memory of Secretary-General Dag Hammarskjöld. The Japanese Peace Bell is just north of the building, and a grove of sycamore trees is planted next to the Secretariat Building. On the western part of the site, along First Avenue, are the flags of the UN, its member states, and its observer states.

Outside of the UN headquarters, Robert Moses Playground is directly to the south, and Tudor City and the Ford Foundation Center for Social Justice are to the west. In addition, One and Two United Nations Plaza (including the Millennium Hilton New York One UN Plaza hotel) are to the northwest. The building is physically isolated from other nearby structures, with the nearest New York City Subway station being several blocks away. Because of this, the Secretariat Building appears as a freestanding tower.

Historically, the site was part of a cove called Turtle Bay. The cove, located between what is now 45th and 48th Streets, was fed by a stream that ran from the present-day intersection of Second Avenue and 48th Street. A creek from the southern end of modern-day Central Park also drained into Turtle Bay. The first settlement on the site was a tobacco farm built in 1639. The site was developed with residences in the 19th century. Slaughterhouses operated on the eastern side of First Avenue for over a hundred years until the construction of the United Nations headquarters. The UN purchased the site in 1946 under the sole condition that it could never slaughter cattle on the land.

The Secretariat Building was designed in the International Style by a team of ten architects working under planning director Wallace K. Harrison. The Board of Design comprised Nikolai Bassov of the Soviet Union; Gaston Brunfaut of Belgium; Ernest Cormier of Canada; Le Corbusier of France; Liang Seu-cheng of China; Sven Markelius of Sweden; Oscar Niemeyer of Brazil; Howard Robertson of the United Kingdom; G. A. Soilleux of Australia; and Julio Vilamajó of Uruguay. Abel Sorenson was the interior designer. In addition, David Fine of United States Steel oversaw the construction of the Secretariat Building. The building houses the administrative functions of the UN, including day-to-day duties such as finance and translation. It contains three basement levels and 39 above-ground stories. When the building was completed, it was cited as measuring 544 ft (166 m) tall, although Emporis and The Skyscraper Center both cite the height as 505 ft (154 m).