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.

The 2020 United States census was the 24th decennial United States census. Census Day, the reference day used for the census, was April 1, 2020. Other than a pilot study during the 2000 census, this was the first U.S. census to offer options to respond online or by phone, in addition to the paper response form used for previous censuses.