Link light rail is a light rail system with some rapid transit characteristics that serves the Seattle metropolitan area in the U.S. state of Washington. It is managed by Sound Transit in partnership with local transit providers, and comprises three non-connected lines that total 47 miles (76 km) with 45 stations. These lines are the 1 Line (formerly Central Link) in King County and Snohomish County, which travels for 33 miles (53 km) between Lynnwood, Seattle, and Seattle–Tacoma International Airport; the 2 Line in King County's Eastside region, which travels for 10 miles (16.1 km) between Bellevue and Redmond; and the T Line (formerly Tacoma Link) in Pierce County, a streetcar which runs for 4 miles (6.4 km) between Tacoma Dome Station, Downtown Tacoma, and Hilltop. In 2024, the system had a ridership of 30.8 million, or about 115,900 per weekday as of the second quarter of 2025, primarily on the 1 Line. Trains run at frequencies of 8 to 20 minutes.

The Link light rail system was originally conceived in the 1980s following several earlier proposals for a heavy rail subway system that were rejected by voters. Sound Transit was created in 1993 and placed a ballot measure to fund and build the system, which was passed on its second attempt in 1996. Tacoma Link began construction first in 2000 and opened on August 22, 2003, becoming the first modern light rail system in the state. Central Link construction in Seattle was delayed because of funding issues and routing disputes, but began in November 2003 and was completed on July 18, 2009. The trains initially ran from Downtown Seattle to Tukwila International Boulevard station before being extended south to Seattle–Tacoma International Airport in December 2009. Further extensions north to the University of Washington and south to Angle Lake station opened in 2016 to complete most of the line's original planned route. An extension from the University of Washington to Northgate station opened on October 2, 2021, followed by a northern extension to Lynnwood City Center station on August 30, 2024.

The first phase of the 2 Line opened on April 27, 2024, between South Bellevue and Redmond Technology stations; an extension east to Downtown Redmond opened in May 2025. It is scheduled to be extended west to Seattle in early 2026 following construction delays on a section crossing Lake Washington. The 2 Line and Lynnwood sections were funded by Sound Transit 2 (ST2), a 2008 ballot measure to expand the transit system, along with planning work for other projects. The Sound Transit 3 (ST3) ballot measure was approved in 2016 and funds plans to expand network to 116 miles (187 km) and 83 stations by 2044. A southern extension of the 1 Line to Federal Way is scheduled to open in late 2025 using a mix of ST2 and ST3 funding. Later projects will expand the system to cover the metropolitan area from Everett to Tacoma, along with branches to Kirkland, Issaquah, and the Seattle neighborhoods of Ballard and West Seattle.

The first form of scheduled public transportation in the Puget Sound region were various steamboat ferries that later evolved into the "mosquito fleet" in the 1880s as the area's population grew. They were followed by streetcar systems, beginning with Seattle's first horse-drawn line in 1884 and Tacoma's in 1888, built by private companies alongside conventional railroads. Electric streetcars and cable cars debuted in 1889 under other companies and continued to grow outward into new developments called streetcar suburbs. The lines were consolidated into the Seattle Municipal Street Railway in 1919; the municipal system grew to 26 routes with 231 miles (372 km) of track, along with 18 gasoline bus routes by 1936. Interurban railways that connected Seattle to Everett and Tacoma were built by Stone & Webster in 1902 and 1910, respectively; they both ceased operations by 1939 due to competition from automobiles on new highways as well as financial issues. Everett's streetcar system was among the first in the United States to be replaced by buses, in 1923, and was followed by Tacoma in 1938 and Seattle in 1941.

Attempts to finance the construction of a rapid transit system to connect Seattle's neighborhoods, and later other cities in the region, first emerged with civil engineer Virgil Bogue's comprehensive plan in 1911. A referendum on the plan, which included 33 miles (53 km) of subway tunnels and 27 miles (43 km) of elevated tracks, was rejected by city voters the following year. The Puget Sound region's transit system faced chronic underinvestment and declining bus ridership as new highways and freeways, including Interstate 5 through Seattle, were constructed to serve a growing number of automobile commuters and address traffic congestion on local streets. A proposal for a 47-mile (76 km) Seattle subway system with suburban extensions to Bellevue and Renton was part of the Forward Thrust series of civic ballot measures in the 1960s. It would use federal funding to cover two-thirds of the $1.15 billion (equivalent to $7.89 billion in 2024 dollars) construction cost, contingent on the approval of local funding, and be completed by 1985. The transit ballot measure, in February 1968, was approved by 51 percent of voters but failed to reach the 60 percent threshold required by the state government to issue municipal bonds. A second vote in May 1970 received only 46 percent approval due to the "Boeing bust", a local economic downturn caused by layoffs at Boeing, a major employer. The federal funding that was earmarked for the Seattle project was instead reallocated to the Metropolitan Atlanta Rapid Transit Authority in Atlanta, Georgia.

The Seattle Transit System, which operated buses within the city, launched one of the nation's first express bus systems in 1970 and drew new suburban riders but continued to face financial issues. King County voters approved a ballot measure in 1972 to acquire the Seattle Transit System and a private suburban operator to form a new countywide bus system that would be managed by the Municipality of Metropolitan Seattle, an existing water treatment and sewage agency. The bus system, named Metro Transit (now King County Metro), began operations in January 1973 and adopted plans to expand its network of express buses, build new park-and-ride facilities, and replace its fleet with modern buses. Annual bus ridership increased from 30 million with Seattle Transit in 1971 to 66 million under Metro Transit in 1980 after service improvements and a refurbishment of the trolleybus network. Metro Transit's buses had difficulty navigating through Downtown Seattle, where 420 trips during peak periods led to congestion in bus lanes and at stops. The Downtown Seattle Transit Tunnel, with five stations on 3rd Avenue and Pine Street, was opened in September 1990; it would redirect bus trips from downtown streets but was also designed to be converted to future light rail use.

In 1975, Metro began a long-term study into regional transit that was later merged with an effort from the Puget Sound Council of Governments (PSCOG), the region's designated metropolitan planning organization. A grant for the study from the Urban Mass Transit Administration was instead allocated to TriMet in Portland, Oregon, due to the federal government's reluctance to lose an "example all-bus system" like Metro Transit's. The regional transit study proceeded and their initial findings in 1981 recommended a light rail system that would be able to operate on streets in some areas but generally follow freeway corridors to regional centers. A related study from the Snohomish County Transportation Authority began in 1983 and determined an extension of a future light rail line along Interstate 5 would be feasible. The Metro–PSCOG study, published and adopted in 1986, recommended a 60-mile (97 km) light rail system with three lines from Downtown Seattle that followed freeway corridors: along Interstate 5 north to Lynnwood and south to Federal Way; and east across Lake Washington to Bellevue. It would be constructed by 2020 if approved by voters and cost approximately $14 billion (equivalent to $33.9 billion in 2024 dollars) to construct.

A non-binding advisory ballot measure on accelerated development of a light rail system was approved by King County voters in November 1988. The state legislature authorized the creation of a regional transit board that would have representatives from King, Pierce, and Snohomish counties. Another study resulted in a long-range plan for a three-county transit system with 105 miles (169 km) of light rail from Everett to Tacoma and Redmond, along with commuter rail and express buses. The Central Puget Sound Regional Transit Authority (RTA) was formed in September 1993 after approval from the state legislature and the three county councils; the RTA would create a construction and financing plan for approval by voters in a regional ballot measure. Their first attempt, which included a 70-mile (110 km) light rail network with lines as far north as Lynnwood and as far south as Tacoma, had an estimated cost of $6.7 billion (equivalent to $12.6 billion in 2024 dollars). It was rejected by 53 percent of voters on March 14, 1995.