Sound Transit Express (ST Express) is a network of regional express buses, operated by the multi-county transit agency, Sound Transit. The routes connect major regional hubs throughout 53 cities in three counties (King, Pierce, and Snohomish) in the Puget Sound region. Sound Transit Express ranks first in the nation in the number of commuter bus passengers carried and in vehicle miles driven. The first nine routes and 114 buses began carrying passengers on September 19, 1999. In 2024, the system had a ridership of 9,325,400, or about 33,500 per weekday as of the second quarter of 2025.

Unlike a typical transit bus, Sound Transit Express routes typically make limited stops as they travel longer distances on the freeways. Most routes operate seven days a week, with runs throughout the day. Where available, buses use transit-only lanes, high-occupancy vehicle lanes, high-occupancy toll lanes, express lanes, bus bypass shoulders, and direct access ramps to speed travel times.

While Sound Transit oversees, plans, and funds the service, operation and maintenance of the buses is contracted out to Community Transit (who subcontracts with Transdev), King County Metro and Pierce Transit.

ST Express uses a flat fare system, where each ride costs the same regardless of distance.

The fares are as follows:

Paper transfers are not accepted or issued on Sound Transit routes. Passengers who use ORCA may transfer between ST routes or routes operated by most other agencies within two hours of initial payment. If the fare for the second route is higher, the difference will be charged.

23 Sound Transit Express bus routes are overseen by the agency. Buses are operated under contract by King County Metro, Pierce Transit and Community Transit (who subcontracts with Transdev). When Sound Transit implements a new bus route, changes are frequently made to existing routes that serve the area to avoid overlapping.

The ST Express routes and operators as of September 14, 2024^[update] are: