State Route 518 (SR 518) is a 3.42-mile-long (5.50 km) state highway located entirely within King County in the U.S. state of Washington. The freeway begins in Burien at an interchange with SR 509 and travels east to serve Seattle–Tacoma International Airport and intersect SR 99 in SeaTac. SR 518 continues east into Tukwila and ends at an interchange with Interstate 5 (I-5), as the freeway becomes I-405. The highway was codified in 1937 as the western segment of Secondary State Highway 1L (SSH 1L) and designated as SR 518 during the 1964 highway renumbering. During the 1970s, the roadway was widened into a freeway to serve the airport and serves over 100,000 vehicles per day.

Burien is a suburban city in King County, Washington, United States, located south of Seattle on Puget Sound. As of the 2020 census, Burien's population was 52,066, which is a 56.3% increase since incorporation in 1993, making it the 25th most populous city in Washington. An annexation in 2010 increased the city's population significantly.