The Washington Metro, commonly called Metro, and unofficially Metrorail, is the rapid transit system in Washington, D.C., United States, and its surrounding suburbs. It is administered by the Washington Metropolitan Area Transit Authority (WMATA), which also operates Metrobus service under the Metro name. In Maryland, Metro provides service to Montgomery County and Prince George's County; in Virginia, to Fairfax County, Arlington County, and the City of Alexandria. Since opening in 1976, the network has grown to include five lines, 86 stations, and 106.3 miles (171.1 km) of track.

Metro is the second-busiest rapid transit system in the United States in number of passenger trips, after the New York City Subway. There were 215.3 million trips, or 727,684 trips per weekday, on Metro in fiscal year 2008. In June 2008, Metro set a new monthly ridership record with 19,729,641 trips, or 798,456 per weekday. Fares vary based on the distance traveled, the time of day, and the type of card used by the passenger. Riders enter and exit the system using a stored-value card in the form of a paper magnetic stripe farecard or a proximity card known as SmarTrip.