King County Metro, officially the King County Metro Transit Department and often shortened to Metro, is the public transit authority of King County, Washington, which includes the city of Seattle. It is the eighth-largest transit bus agency in the United States. In 2023, the system had a ridership of 78,121,600, or about 256,200 per weekday as of the fourth quarter of 2023. Metro employs 2,444 full-time and part-time operators and operates 1,540 buses.

King County is located in the U.S. state of Washington. The population was 2,269,675 in the 2020 census, making it the most populous county in Washington, and the 12th-most populous in the United States. The county seat is Seattle, also the state's most populous city.