SR 203 begins at a roundabout with SR 202 on the north side of the Snoqualmie River opposite from downtown Fall City. The highway travels northwest along the river, following the eastern ridge of the rural Snoqualmie Valley and the Snoqualmie Valley Trail, and turns north near Pleasant Hill. After passing several farms and a golf course, SR 203 crosses over the Tolt River and enters the city of Carnation. The highway runs northeasterly through downtown Carnation on Tolt Avenue, passing the city library and the historic Tolt IOOF/Eagles Hall before leaving the city. After crossing the Snoqualmie Valley Trail at the Chinook Bend north of Carnation, SR 203 reaches Lake Marcel and turns northwest to follow the Snoqualmie River around Novelty Hill.