King County Metro, officially the King County Metro Transit Department and often shortened to Metro, is the public transit authority of King County, Washington, which includes the city of Seattle. It is the eighth-largest transit bus agency in the United States. In 2024, the system had a ridership of 88,902,700, or about 302,900 per weekday as of the second quarter of 2025. Metro employs 2,477 full-time and part-time operators and operates 1,540 buses.

King County Metro formally began operations on January 1, 1973, but can trace its roots to the Seattle Transit System, founded in 1939, and Overlake Transit Service, a private operator founded in 1927 to serve the Eastside. Metro is also contracted to operate and maintain Sound Transit's 1 Line Link light rail line and eight of the agency's Sound Transit Express bus routes along with the Seattle Streetcar lines owned by the City of Seattle. Metro's services include electric trolleybuses in Seattle, RapidRide enhanced buses on eight lines, commuter routes along the regional freeway system, dial-a-ride routes, paratransit services, and overnight “owl” bus routes.

A horse-drawn streetcar rail system debuted in Seattle in 1884 as the Seattle Street Railway. In 1918, the city of Seattle bought many parts of the Seattle Street Railway, on terms which left the transit operation in financial trouble.

In 1939, a new transportation agency, the Seattle Transit System, was formed, which refinanced the remaining debt and began replacing equipment with "trackless trolleys" (as they were known) and motor buses. The final streetcar ran on April 13, 1941.

The Municipality of Metropolitan Seattle was created by a local referendum on September 9, 1958, as a regional authority tasked with management of wastewater and water quality issues in King County. The authority was formed after civic leaders, including those in the Municipal League, noted that solutions to regional issues were complicated by local boundaries and a plethora of existing special districts. The state legislature approved the formation of a combined transportation, sewage, and planning authority in 1957, but the countywide referendum was rejected by a majority outside of Seattle. Metro, as the authority came to be called, was restricted to sewage management and given a smaller suburban jurisdiction ahead of the successful September referendum. By 1967, the agency had completed its $125 million sewage treatment system, which diverted 20 million gallons (76 million liters) that had previously contaminated Lake Washington.

After two failed attempts to enable it to build a regional rapid transit system, it was authorized to operate a regional bus system in 1972. The bus system was known as Metro Transit and began operations on January 1, 1973; other suggested names included King Area Rapid Transit (KART) and Seattle Metropolitan Area Rapid Transit (SMART). Its operations subsumed the Seattle Transit System, formerly under the purview of the City of Seattle and the Metropolitan Transit Corporation, a private company serving suburban cities in King County. In the early 1970s, the private Metropolitan faced bankruptcy because of low ridership. King County voters authorized Metro to buy Metropolitan and operate the county's mass transit bus system.^{[citation needed]} Metro Transit introduced its new services in September 1973, including a ride-free area in downtown and express routes on freeways (known as "Flyer" routes), and a unified numbering scheme in 1977 that replaced named routes. The agency introduced its first paratransit service in 1979, which was followed by buses equipped with wheelchair lifts in 1980.

To address bus congestion in Downtown Seattle and prepare for a regional rapid transit system, Metro Transit proposed the construction of the Downtown Seattle Transit Tunnel in the early 1980s. Excavation of the 1.3-mile (2.1 km) tunnel began in 1987 and was completed on September 15, 1990, with five stations under 3rd Avenue and Pine Street. The project cost $455 million to construct and used a fleet of 236 dual-mode diesel–electric buses manufactured in Italy by Breda. Buses on commuter routes would switch to electric trolleybus wires within the tunnel, which was also designed for future use by light rail trains.

The Municipality of Metropolitan Seattle was overseen by a federated board of elected officials, composed of elected officials from cities throughout the region. Its representation structure was ruled unconstitutional in 1990 on the grounds of "one person, one vote" following a similar ruling in Board of Estimate of City of New York v. Morris. In 1992, after gaining approval by popular vote, the municipality's roles and authorities were assumed by the government of King County. The municipality's transit operations was a stand-alone department within the county until 1996, when it became a division of the newly created King County Department of Transportation. In August 2018, the county council approved legislation to separate Metro from the Department of Transportation, creating the King County Metro Transit Department effective January 1, 2019.