Located in the counties of Lewis and Pierce, the 16.34-mile (26.30 km) long roadway extends through a heavily forested canyon from U.S. Route 12 (US 12) to SR 410.

The northern terminus of the highway, Cayuse Pass, is closed annually and in late 2006, the Hanukkah Eve windstorm of 2006 washed out a 10.90-mile (17.54 km) long segment of the roadway.

Paralleling US 12 and the Ohanapecosh River, the highway crosses Boulder Creek and enters a canyon where the roadway encounters Forest Route 44 (FR 44), a connector from SR 706 to Mount Rainier National Park.

SR 123 crosses Laughingwater Creek and passes Silver Falls and the entrance to Steve Canyon to the trailhead of the Grove of the Patriarchs Trail.

Now following the Chinook Creek, the highway travels into a tunnel under Seymour Peak and ends at an intersection with SR 410 at Cayuse Pass.

[13][14] The northern terminus of the roadway at Cayuse Pass became U.S. Route 410 (US 410) during the creation of the United States Numbered Highways.