State Route 18 (SR 18) is a 28.41-mile-long (45.72 km) state highway in the U.S. state of Washington, serving southeastern King County. The highway travels northeast, primarily as a controlled-access freeway, from an intersection with SR 99 and an interchange with Interstate 5 (I-5) in Federal Way through the cities of Auburn, Kent, Covington, and Maple Valley. SR 18 becomes a two-lane rural highway near Tiger Mountain as it approaches its eastern terminus, an interchange with I-90 near the cities of Snoqualmie and North Bend.

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.