Southeast is the southeastern quadrant of Washington, D.C., the capital of the United States, and is located south of East Capitol Street and east of South Capitol Street. It includes the Capitol Hill and Anacostia neighborhoods, the Navy Yard, the Joint Base Anacostia-Bolling (JBAB), the U.S. Marine Barracks, the Anacostia River waterfront, Eastern Market, the remains of several Civil War-era forts, historic St. Elizabeths Hospital, RFK Stadium, Nationals Park, and the Congressional Cemetery. It also contains a landmark known as "The Big Chair," located on Martin Luther King Jr. Avenue. The quadrant is split by the Anacostia River, with the portion that is west of the river sometimes referred to as "Near Southeast". Geographically, it is the second-smallest quadrant of the city.

Washington, D.C., is administratively divided into four geographical quadrants of unequal size, each delineated by their ordinal directions from the medallion located in the Crypt under the Rotunda of the Capitol. Street and number addressing, centered on the Capitol, radiates out into each of the quadrants, producing a number of intersections of identically named cross-streets in each quadrant. Originally, the District of Columbia was a near-perfect square but contained more than one settlement; the Capitol was to be the center of the City of Washington. Thus, the Capitol was never located at the geographic center of the whole territory, which was eventually north of the Potomac River, consolidated into one city. As a result, the quadrants are of greatly varying size. Northwest is quite large, encompassing over a third of the city's geographical area, while Southwest is little more than a few neighborhoods, large parks, and a military base.