The King County Sheriff's Office (KCSO) is a local law enforcement agency in King County, Washington, United States. It is the primary law enforcement agency for all unincorporated areas of King County, as well as 12 cities and two transit agencies which contract their police services to the KCSO. KCSO also provides police and fire Aircraft Rescue and Fire Fighting to the King County International Airport. KCSO also provides regional-level support services to other local law enforcement agencies such as air support and search and rescue. The department has over 1,000 employees and serves 2.1 million citizens, over 500,000 of whom live in either unincorporated areas or the 12 contract cities.

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.