SR 27 begins its 90-mile-long (145 km) route as Grand Avenue and part of the Palouse Scenic Byway at an intersection with US 195 south of Pullman.

[3][4] The highway travels through downtown Pullman and becomes concurrent with SR 270 as it passes west of the Washington State University main campus.

[8] The highway continues north and crosses two rail lines, belonging to the Spokane International branch of Union Pacific and BNSF,[6] before the designation ends at Trent Avenue, signed as SR 290.

[10][11] The branch route traveled 68.71 miles (110.58 km) north from US 195 at the Idaho state border through Pullman and Oakesdale to PSH 3 and US 195 south of Rosalia.

[26] The northern terminus of SR 27 is planned to be rebuilt by the Spokane Valley government as part of a railroad grade separation project beginning in 2024.

The rebuilt highway would travel under the BNSF Railway and curve east before reaching a roundabout with SR 290 adjacent to the Spokane River.