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, in the Allentown neighborhood of northern Tukwila. 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 roadway continues southeast into Skyway whilst parallel to I-5 and the BNSF South Seattle railyard. SR 900 turns east and becomes Sunset Boulevard as it enters Renton, following the Black River and a BNSF branch railroad, to an intersection with SR 167 at Rainier Avenue.