The Downtown Seattle Transit Tunnel (before Link Light Rail, called the Metro Bus Tunnel) is a 1.3 mile tunnel beneath downtown Seattle. Until buses were kicked out of the tunnel on March 23, 2019 (to allow increases in Link Light Rail frequencies) 18 King County Metro and one Sound Transit Express bus routed use the tunnel. These are all Express routes that run via freeways on each end of their tunnel trips. The Central Link Light Rail also used the tunnel but only as far as Westlake Station. The tunnel has a total of 5 stations. All bus routes served all five stations before terminating at the northern end or southern end of the tunnel. The three middle ones are underground, Convention Place is just below grade and in the middle of a large layover area, and International District/Chinatown is in an open cut, a good portion of which has been covered over.

The tunnel began construction in 1987 and opened in 1990 for buses. It had to be closed for exactly two years from September 24, 2005 to September 24, 2007 to retrofit it for light rail service that mostly consisted of lowering the roadway/track bed for the trains and replacing the trolleybus wire with light rail wire. The tunnel opened with tracks for future light rail already in place but this was designed incorrectly with the tracks not built in a way that they could have ever been usable. Light rail service finally began on July 18, 2009.

For propulsion from the tunnels opening in 1990 to 2005 buses used dual-mode buses running via trolley wire propulsion through the tunnels before converting to a conventional diesel engine for their runs to their far away express destinations (similar to the MBTA Silver Line today). Although all run entirely at the surface King County Metro still has 14 full electric trolley bus routes serving 69 miles of two-way street wire. A few of the former dual-mode Breda ETB Buses were converted to become all electric buses to operate on the trolley bus routes.

From 2007 through 2019, bus routes through the tunnel used New Flyer DE60LF hybrid buses all of which are articulated that run in Hush Mode while in the tunnel, operating on electricity from the batteries on board only running the diesel engine sparingly to recharge the batteries.