State Route 509 (SR 509) is a 35.17-mile-long (56.60 km) state highway in the U.S. state of Washington, connecting Tacoma in Pierce County to Seattle in King County. The highway travels north from Interstate 705 (I-705) in Tacoma to SR 99 south of downtown Seattle. It serves cities along the Puget Sound and west of Seattle–Tacoma International Airport in south King County, including Federal Way, Des Moines, and Burien.