After approximately 3 miles (5 km), the highway crosses over the Grande Ronde River and enters the bottom of Rattlesnake Canyon, which it follows northeast towards a fork in the stream.

The highway then travels northeast along Mill Creek and Ayers Gulch, making a brief turn due east near Dwight Halsey Road before continuing north towards Asotin.

On the southern outskirts of the city, SR 129 begins a series of switchback turns while descending from the plateau towards Hells Canyon on the Snake River.

[3][4] SR 129 enters Asotin near the county fairgrounds and meanders around a residential neighborhood before turning north onto Washington Street at the east end of downtown.

The highway continues northeast along the river bank and a parallel bicycle path towards Clarkston,[8] passing downhill from residential areas on the west and the Lewiston–Nez Perce County Airport to the east in Idaho.

[3][4] At the south end of Clarkston near Swallows Park, SR 129 intersects Fleshman Way in a dogbone interchange on the western approach to the Southway Bridge.

Average traffic volumes on SR 129 in 2016 ranged from a minimum of 180 vehicles near the Oregon state line to a maximum of 9,800 in downtown Clarkston.

[19] The Southway Bridge between Clarkston and Lewiston was completed in 1981 by the U.S. Army Corps of Engineers and is maintained by the Asotin and Nez Perce county governments.