The Orange Line is one of the six rapid transit lines of the Washington Metro system, consisting of 26 stations in Fairfax County and Arlington in Northern Virginia; Washington, D.C.; and Prince George's County, Maryland, United States. The Orange Line runs from Vienna in Fairfax County to New Carrollton in Prince George's County. Half of the line's stations are shared with the Blue Line and all but three stations (Vienna, Dunn Loring, and West Falls Church) are shared with the Silver Line. Orange Line service began on November 20, 1978.

Trains run every 10 minutes during weekday rush hours, every 12 minutes during weekday off-peak hours and weekends, and every 15 minutes daily after 9:30 pm.

In 1955, planning for Metro began with the Mass Transportation Survey, which attempted to forecast both freeway and mass transit systems sufficient to meet the needs of transportation in 1980. In 1959, the study's final report included two rapid transit lines which anticipated subways in downtown Washington. Because the plan called for extensive freeway construction within Washington, D.C., alarmed residents lobbied for federal legislation creating a moratorium on freeway construction through July 1, 1962. The National Capital Transportation Agency's 1962 Transportation in the National Capital Region report anticipated much of the present Orange Line route in Virginia with the route following the median strip of Interstate 66 both inside Arlington and beyond. The route continued in rapid transit plans until the formation of the Washington Metropolitan Area Transit Authority (WMATA).

With the formation of WMATA in October 1966, planning of the system shifted from federal hands to a regional body with representatives of the District, Maryland and Virginia. Congressional route approval was no longer a key consideration. Instead, routes had to serve each local suburban jurisdiction to assure that they would approve bond referendums to finance the system. Because the least expensive way to build into the suburbs was to rely upon existing railroad right-of-ways, the Orange Line took much of its present form, except that it also featured a further extension along the railroad to Bowie, Maryland and along the Dulles Access Road to Dulles International Airport.

By 1966, WMATA and Arlington County planners had agreed "to realign the rapid transit through high-density commercial-office-apartment areas in the vicinity of Wilson Boulevard instead of the freeway's median between the river and Glebe Road." As a result of this agreement, the Orange Line follows in Arlington the former routes of an interurban electric trolley line, the Fairfax line and the North Arlington branch of the Washington, Arlington & Falls Church Railway, that had initially spurred those areas' development.

In March 1968, WMATA approved its Adopted Regional System (ARS) plan that included suburban mass transit lines that followed the median of the proposed Interstate 66 through Virginia to Vienna and the CSX/Amtrak railroad right-of-way in Prince George's County, Maryland. The construction of the downtown Washington sections of the Orange and Blue lines began simultaneously with the Red line. A joint ground-breaking ceremony was held on December 9, 1969. Service on the joint downtown track was at first branded as just the Blue Line and commenced on July 1, 1977.

In 1976, Robert Patricelli, federal Urban Mass Transportation Administrator, ordered Metro to conduct an alternatives analysis of the portion of its system that was not already under contract. Because the Tysons area of Fairfax County developed significantly since the ARS was adopted in 1968, the analysis considered rerouting the Orange line to serve Tysons Corner at an additional cost of $60 million. However, because environmental impact statements had already been completed for the Vienna route, a change in the route would result in a five-year delay in the construction of the Orange Line west of Ballston. This prompted the City of Falls Church to sue WMATA for breach of contract. In the end, WMATA kept the Vienna route intact, leaving Tysons without Metrorail service until 2014.

Service on the Orange Line began on November 20, 1978 between National Airport and New Carrollton, with five new stations being added to the existing network from Stadium–Armory. When the line from Rosslyn to Ballston-MU was completed on December 11, 1979, Orange Line trains began following this route rather than going to the National Airport station. The line was completed on June 7, 1986, when it was extended by four stations to Vienna in the I-66 median.