The King County Library System (KCLS) is a library system serving most residents of King County, Washington, United States. Headquartered in Issaquah, Washington, KCLS was the busiest library system in the United States as of 2010, circulating 22.4 million items. It consists of 50 libraries, a Traveling Library Center, a mobile TechLab, and the ABC Express children’s library van. KCLS offers a collection of more than 4.1 million items, including books, periodicals, newspapers, audio and videotapes, films, CDs, DVDs and extensive online resources. All KCLS libraries offer free Wi-Fi connections. Patrons can check out 100 items at once and hold up to 50 items.

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.