Washington, D.C. (pronounced /ˈwɒʃɪŋtən ˌdiːˈsiː/), formally the District of Columbia and commonly referred to as Washington, the District, or simply D.C., is the capital of the United States, founded on July 16, 1790. The City of Washington was originally a separate municipality within the Territory of Columbia until an act of Congress in 1871 effectively merged the City and the Territory into a single entity called the District of Columbia. It is for this reason that the city, while legally named the District of Columbia, is known as Washington, D.C. The city is located on the north bank of the Potomac River and is bordered by the states of Virginia to the southwest and Maryland to the other sides. The District has a resident population of 591,833; however, because of commuters from the surrounding suburbs, its population rises to over one million during the workweek. The Washington Metropolitan Area, of which the District is a part, has a population of 5.3 million, the eighth-largest metropolitan area in the country.

Article One of the United States Constitution provides for a federal district, distinct from the states, to serve as the permanent national capital. The centers of all three branches of the federal government of the United States are located in the District, as are many of the nation's monuments and museums. Washington, D.C., hosts 174 foreign embassies as well as the headquarters of the World Bank, the International Monetary Fund (IMF), the Organization of American States (OAS), the Inter-American Development Bank, and the Pan American Health Organization (PAHO). The headquarters of other institutions such as trade unions, lobbying groups, and professional associations are also located in the District.

The United States Congress has supreme authority over Washington, D.C.; residents of the city therefore have less self-governance than residents of the states. The District has a non-voting at-large Congressional delegate, but no senators. D.C. residents could not vote in presidential elections until the ratification of the Twenty-third Amendment to the United States Constitution in 1961.

The National Mall is a large, open park area in the center of the city. Located in the center of the Mall is the Washington Monument. Also located on the mall are the Lincoln Memorial, the National World War II Memorial at the east end of the reflecting pool, the Korean War Veterans Memorial, the Vietnam Veterans Memorial, and the Albert Einstein Memorial. The National Archives houses thousands of documents important to American history including the Declaration of Independence, the United States Constitution, and the Bill of Rights.

Located directly south of the mall, the Tidal Basin features rows of Japanese cherry blossom trees that were presented as gifts from the nation of Japan. The Franklin Delano Roosevelt Memorial, Jefferson Memorial, and the District of Columbia War Memorial are located around the Tidal Basin.

The Smithsonian Institution is an educational foundation chartered by Congress in 1846 that maintains most of the nation's official museums and galleries in Washington, D.C. The U.S. government partially funds the Smithsonian, thus making its collections open to the public free of charge. The most visited of the Smithsonian museums in 2007 was the National Museum of Natural History located on the National Mall. Other Smithsonian Institution museums and galleries located on the mall are: the National Air and Space Museum; the National Museum of African Art; the National Museum of American History; the National Museum of the American Indian; the Sackler and Freer galleries, which both focus on Asian art and culture; the Hirshhorn Museum and Sculpture Garden; the Arts and Industries Building; the S. Dillon Ripley Center; and the Smithsonian Institution Building (also known as "The Castle"), which serves as the institution's headquarters.

The Smithsonian American Art Museum (formerly known as the National Museum of American Art) and the National Portrait Gallery are located in the same building, the Donald W. Reynolds Center, near Washington's Chinatown. The Reynolds Center is also known as the Old Patent Office Building. The Renwick Gallery is officially part of the Smithsonian American Art Museum but is located in a separate building near the White House. Other Smithsonian museums and galleries include: the Anacostia Community Museum in Southeast Washington; the National Postal Museum near Union Station; and the National Zoo in Woodley Park.

The National Gallery of Art is located on the National Mall near the Capitol, but is not a part of the Smithsonian Institution. It is instead wholly owned by the U.S. government; thus admission to the gallery is free. The gallery's west wing features the nation's collection of American and European art through the 19th century. The east wing, designed by architect I. M. Pei, features works of modern art. The Smithsonian American Art Museum and the National Portrait Gallery are often confused with the National Gallery of Art when they are in fact entirely separate institutions. The National Building Museum, located near Judiciary Square, was chartered by Congress and hosts temporary and traveling exhibits.

There are many private art museums in the District of Columbia, which house major collections and exhibits open to the public such as: the National Museum of Women in the Arts; the Corcoran Gallery of Art, the largest private museum in Washington; and The Phillips Collection in Dupont Circle, the first museum of modern art in the United States. Other private museums in Washington include the Newseum, the International Spy Museum, the National Geographic Society museum, and the Marian Koshland Science Museum. The United States Holocaust Memorial Museum located near the National Mall maintains exhibits, documentation, and artifacts related to the Holocaust.