The Central Puget Sound Regional Transit Authority, branded as Sound Transit (ST), is a public transit agency serving the Seattle metropolitan area in the U.S. state of Washington. It manages the Link light rail system in Seattle and Tacoma, regional Sounder commuter rail, and Sound Transit Express bus service. The agency also coordinates with the regional ORCA fare card system used by transit operators across the metropolitan area. In 2024, Sound Transit services carried a total of 42 million passengers and averaged over 134,000 riders on weekdays.

The agency was created in 1993 by King, Pierce and Snohomish counties to build a regional rapid transit system. After an unsuccessful proposal in 1995, the "Sound Move" plan for regional light rail, commuter rail, and express bus service was approved by voters in November 1996. Sound Transit began operating its express bus service in September 1999, taking over existing routes from local transit agencies. The region's first commuter rail line, between Tacoma and Seattle, started in December 2000; the agency's first light rail line, Tacoma Link (now the T Line), began service in August 2003. Light rail service in Seattle on Central Link (now the 1 Line) began in 2009, and is the largest part of the Sound Transit system in terms of ridership. Union Station in Seattle has served as the agency's headquarters since its renovation in 1999.

Sound Transit is independent of local transit agencies and is governed by an eighteen-member board of directors, which consists of elected officials from member jurisdictions and the Secretary of Transportation. It is funded by local sales taxes, property taxes, and motor vehicle excise taxes levied within its taxing district in portions of King, Pierce and Snohomish counties. The agency has passed three major ballot measures to fund system expansion: Sound Move (1996), Sound Transit 2 (2008) and Sound Transit 3 (2016). Planning and construction of transit projects is scheduled to continue until 2041 under the Sound Transit 3 plan, which would expand the light rail network to 116 miles (187 km) and 83 stations.

Sound Transit operates three main transit services across the Seattle metropolitan area: the Link light rail system, which serves as the rapid transit system for the region; the Sounder commuter rail system from Everett to Lakewood, via Seattle; and the Sound Transit Express bus system across the three counties. In 2024, these systems carried more than 42 million total passengers, averaging 132,904 riders per day on weekdays. A fourth service, Stride bus rapid transit, is planned to be introduced in the late 2020s. Sound Transit's services use a unified livery and paint scheme that consists of blue, teal, and green waves against a white background. Link, Sounder, and Stride all use the same naming scheme for lines and services, which switched from colors to numbers and letters in 2021.

All of Sound Transit's services accept cash payment and mobile tickets as well as the regional ORCA card, a contactless proximity card with stored fares and passes. As of 2024^[update], fares on Link and Sound Transit Express use a flat rate while most Sounder passengers pay based on the traveled distance. Discounts are offered for eligible low-income households, senior citizens, and people with disabilities; passengers under the age of 19 are not charged a fare as part of a statewide program. Sound Transit's light rail and commuter rail stations do not have faregates or turnstiles, instead relying on proof of payment that is enforced by fare ambassadors.

As of 2025^[update], Sound Transit has a fleet of 558 vehicles for use on its services. The fleet is composed of 218 low-floor light rail vehicles for the 1 and 2 Lines; 8 streetcars for the T Line; 78 bilevel cars and 14 locomotives for Sounder; and 240 buses for Sound Transit Express. The entire Link and Sound Transit Express fleet is considered accessible, with level boarding or ramps and lifts. Sounder trains have a high platform with a designated level boarding area for ramps. All vehicles are also equipped to carry bicycles in racks; Sound Transit also provides bicycle cages at some stations. By 2027, the agency plans to deploy an additional 10 light rail vehicles for Link and 44 battery electric buses for the Stride bus rapid transit system, including double-decker buses.

The Link light rail system encompasses three lines with 63 miles (101 km) of track and 50 stations in three counties. The 1 Line and 2 Line are interconnected and share stations between Snohomish County and Seattle. After the two lines split in Seattle, the 1 Line continues south to SeaTac and Federal Way; the 2 Line travels east to Bellevue and Redmond. The T Line is disconnected from the rest of the system and operates entirely within Tacoma. Link trains carried 30.8 million passengers in 2024 and averaged 90,050 passengers on weekdays across all three lines. It is the fourth-busiest light rail system in the United States by ridership, behind Los Angeles, San Diego, and Boston.

Link trains generally run seven days a week at frequencies of 4 to 24 minutes, with stops spaced closely together. All stations are accessible and most offer connections to nearby buses or a park and ride facility. The system is planned to expand to 83 stations and 116 miles (187 km) by 2041, with five lines serving all three counties. 1 Line and 2 Line trains are operated and maintained under contract with King County Metro and are able to use longer, four-car trainsets that have a typical capacity of 150 passengers in each car. The T Line uses low-floor streetcars, unable to be coupled into pairs, and are the only Sound Transit service to be directly operated and maintained by the agency rather than a contractor.