The highway terminates to the west at an interchange with Interstate 5 (I-5) in Tukwila and to the east at I-90 in Issaquah, and also has intermediate junctions with I-405 and SR 167 in Renton.

SR 900 begins as an extension of Martin Luther King Jr. Way South at an intersection with Boeing Access Road, east of its interchange with I-5 under an elevated section of the Central Link light rail line, near the Allentown neighborhood of northern Tukwila.

[3] The four-lane highway splits at a basketweave interchange with I-5, with the southbound lanes of SR 900 traveling over the freeway twice.

The highway transitions into a two-lane country road, entering unincorporated King County while following Tibbetts Creek northeastward between Cougar and Squak mountains, part of the Issaquah Alps highlands.

At the time the highway was the main thoroughfare between Seattle and Spokane, with a route then around the south end of Lake Washington.

After April 1, 1992, the stretch of SR 900 between I-90 exit 3 at Rainier Avenue and along Martin Luther King Jr. Way to the Boeing Access Road was dropped from the officially designated highway.