In the southwest corner of Idaho, State Highway 55 starts at the junction with US-95, approximately two miles (3 km) west of Marsing.

The new multi-lane grade was completed in late 1991;[3] the old curvier road to the west was long plagued by landslides and extended closures.

[4][5][6] North of Horseshoe Bend, the highway ascends the Payette River and passes through several recreational areas, which offer rafting, fishing, and camping activities.

The route continues northward to Donnelly, the turnoff for Tamarack Resort, located to the southwest, on the western shore of the reservoir.

SH-55 continues north through the valley to McCall, at the south shore of Payette Lake, the host of many scenic, recreational, and winter activities.

West of town, the road climbs to the Red Ridge, where it reaches its maximum elevation of 5,324 feet (1,623 m) at the Adams County line, also known as Goose Creek Summit immediately northeast of the Little Ski Hill.