K Street is a major thoroughfare in the United States capital of Washington, D.C. known as a center for numerous think tanks, lobbyists, and advocacy groups. In political discourse, "K Street" has become a metonym for Washington's lobbying industry since many lobbying firms were traditionally located on the section in Northwest Washington which passes from Georgetown through a portion of downtown D.C. Since the late 1980s, however, many of the largest lobbying firms have moved out; as of 2012, only one of the top-20 lobbying firms has a K Street address.
In Washington's street grid there are two unconnected streets designated as K Street. The term "K Street" almost always refers to the northern K Street.
The northern K Street, which carries a segment of U.S. Route 29, begins in the city's Northwest quadrant as K St. NW, just west of the abutment of the old Aqueduct Bridge on the Georgetown waterfront. The street travels east underneath the Whitehurst Freeway, crosses Rock Creek on the K Street Bridge, and continues through downtown D.C. After its intersection with North Capitol Street, the street's designation changes to K Street NE as it enters the Northeast quadrant. The street ends at Florida Avenue in the Near Northeast neighborhood, just south of Gallaudet University. It briefly reappears further east in the Carver neighborhood.
The Georgetown section of the street was known as Water Street prior to the Georgetown street renaming of 1895. West of 33rd Street NW, the United States Postal Service still recognizes both "K Street" and "Water Street" in addresses. The westernmost end of K Street occupies the former right of way of the Georgetown Branch of the Baltimore and Ohio Railroad. When that line was abandoned, K Street was extended west to the Washington Canoe Club. The rest of the Georgetown right of way is now occupied by the Capital Crescent Trail, which begins at the terminus of K Street.
K Street provides a major east-west thoroughfare for traffic through Washington, primarily from Mount Vernon Square to the Whitehurst Freeway. The street continues through Georgetown under the Whitehurst Freeway; however most westbound traffic exits to the freeway. A portion of the street travels in a tunnel underneath Washington Circle, allowing through traffic to avoid the circle. Portions of the street are divided into both "local" (or service) lanes and "express" lanes in both directions. 
The Washington Metropolitan Area Transit Authority has been studying the conversion of K Street into part of a proposed busway. The route would begin at Georgetown University, cross downtown Washington on K Street, switch to Massachusetts Avenue at Mount Vernon Square, and end at Union Station. Currently, the DC Circulator provides service along most of the proposed route, although it must share right of way with other vehicles.
"K Street" is the common and often negative metonym for Washington's lobbying industry.   Many of the major Washington lobbying firms were located on the section in Northwest Washington which passes from Georgetown through a portion of downtown D.C., although the strip has grown less popular for lobbyists in recent years. The major lobbying firms that remain on K Street, as of 2017, include CGCN Group, K&L Gates and Wiley Rein.