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.

SR 509 was re-aligned onto the Burien Freeway in 1968 and the Port of Tacoma bypass in 1997, coinciding with the opening of its interchange with I-705 and the cable-stayed 21st Street Bridge.

[7][8] The highway turns northwest along the Hylebos Waterway as Marine View Drive through the neighborhood of Northeast Tacoma towards Browns Point.

The highway turns southeast onto Ambaum Boulevard and Des Moines Memorial Drive to a trumpet interchange, the southern terminus of a limited-access freeway section of SR 509.

[24] The present route of SR 509 from Des Moines to Seattle roughly follows a wagon road constructed in the late 1890s by King County along the Puget Sound.

[35] An earlier proposal from 1967 to connect the north end of SR 509 to I-5 via an expressway on Michigan Street and a new crossing of the Duwamish River was studied but was never built.

[2] The freeway was planned to be extended south to SR 516 in the 1970s after a 4-mile (6.4 km) right-of-way was acquired by WSDOT before the project was canceled due to public opposition.

[40] The King County Department of Public Works recommended extending SR 509 southeast from South 188th Street to I-5 in 1988, with construction planned to begin in 1999 at an estimated cost of $252 million in 1996,[39][41] raised to $1.4 billion a decade later.

[42] A freeway bypass of the Port of Tacoma for SR 509 was proposed in 1990 as a supplement to the newly completed I-705 and funded with a $180 million grant from the Federal Highway Administration approved the following year.

[46][47] The project cost a total of $165.3 million to construct, with the majority of funds from the federal government and a land claims settlement with the Puyallup Tribe.