Capitol Hill, in addition to being a metonym for the United States Congress, is the largest historic residential neighborhood in Washington, D.C., stretching easterly in front of the United States Capitol along wide avenues. It is one of the oldest residential neighborhoods in Washington, D.C., and, with roughly 35,000 people in just under 2 square miles (5 km²), it is also one of the most densely populated.

As a geographic feature, Capitol Hill rises near the center of the District of Columbia and extends eastward. Pierre (Peter) Charles L'Enfant, as he began to develop his plan for the new federal capital city in 1791, chose to locate the "Congress House" (the Capitol building) on the crest of the hill at a site that he characterized as a "pedestal waiting for a monument." The Capitol building has been the home of the Congress of the United States and the workplace of many residents of the Capitol Hill neighborhood since 1800.

The Capitol Hill neighborhood today straddles two quadrants of the city, Southeast and Northeast. A large portion of the neighborhood is now designated as the Capitol Hill Historic District.

The name Capitol Hill is often used to refer to both the historic district and to the larger neighborhood around it. To the east of Capitol Hill lies the Anacostia River, to the north is the H Street corridor, to the south are the Southeast/Southwest Freeway and the Washington Navy Yard, and to the west are the National Mall and the city's central business district.

The Capitol building is surrounded by the Capitol Hill Historic District, which is listed on the National Register of Historic Places (NRHP). The Capitol Hill Historic District was expanded in 2015 to the north to include the blocks bordered by 2nd Street, F Street, 4th Street, and just south of H Street, NE, collectively known as the Swampoodle Addition.