The Downtown Seattle Transit Tunnel (DSTT), also referred to as the Metro Bus Tunnel, is a 1.3-mile-long (2.1 km) pair of public transit tunnels in Seattle, Washington, United States. The double-track tunnel and its four stations serve Link light rail trains on the 1 Line as it travels through Downtown Seattle. It runs west under Pine Street from 9th Avenue to 3rd Avenue, and south under 3rd Avenue to South Jackson Street. 1 Line trains continue north from the tunnel to Northgate station and south through the Rainier Valley past Seattle–Tacoma International Airport to Angle Lake station as part of Sound Transit's light rail network.

Link light rail is a light rail rapid transit system serving the Seattle metropolitan area in the U.S. state of Washington. It is managed by Sound Transit in partnership with local transit providers, and consists of three non-connected lines: the 1 Line in King County, which travels for 26 miles (42 km) between Seattle and Seattle–Tacoma International Airport; the 2 Line in King County's Eastside region between Bellevue and Redmond; and the T Line in Pierce County, which runs for 4 miles (6.4 km) between Downtown Tacoma and Tacoma Dome Station. In 2023, the system had a ridership of 23.9 million, or about 78,600 per weekday as of the fourth quarter of 2023, primarily on the 1 Line. Trains run at frequencies of 6 to 24 minutes.