The headquarters building of the US Department of Defense, located in Arlington County, Virginia, across the Potomac River from Washington, D.C.

The Pentagon is the world's largest office building, with about 6.5 million square feet (150 acres; 60 ha) of floor space, of which 3.7 million square feet (85 acres; 34 ha) are used as offices.

Some 23,000 military and civilian employees, and another 3,000 non-defense support personnel, work in the Pentagon. It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 mi (28.2 km) of corridors. The central five-acre (2.0 ha) pentagonal plaza is nicknamed "ground zero" on the presumption that it would be a prime target in a nuclear war.

The building was designed by American architect George Bergstrom and built by contractor John McShain. Ground was broken on 11 September 1941, and the building was dedicated on 15 January 1943. General Brehon Somervell provided the major impetus to gain Congressional approval for the project; Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army Corps of Engineers, which supervised it.