The highway splits into a couplet on 2nd and 3rd streets for several blocks in downtown Renton, serving the Renton transit center, before returning to two-way traffic at a bridge over the Cedar River, downstream of the Renton branch of the King County Library System. SR 900 then continues east, around the northwest side of Liberty Park, before turning north to briefly parallel I-405 after its interchange with SR 169. The highway then joins I-405 in a short concurrency for , traveling east through Renton's suburbs towards the East Renton Highlands. 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. SR 900 enters Issaquah as the four-lane 17th Avenue, passing the Issaquah park and ride, before ending at a partial cloverleaf interchange with I-90 south of Lake Sammamish State Park.