US-26 runs through the large population centers of Boise and Idaho Falls, as well as the smaller communities of Parma, Notus, Garden City, Shoshone, Richfield, Carey, Arco, Blackfoot, Swan Valley, and Irwin.

It does have significant four-lane segments within the Boise metropolitan area and east of Idaho Falls.

In fact, its first 138.97 miles (223.65 km) in the state consists of concurrencies with any one of five different highways, principally US-20 and Interstate 84 (I-84).

After crossing the Snake River at the Oregon state line, US-26 and US-20 make a curve toward US-95.

Along the way, the freeway serves several state highways, I-84 business loops, and local roads in different locations.

At one point east of Mountain Home, US-20 leaves the freeway and travels northeastward.

The three routes serve Craters of the Moon National Monument and Preserve and Blizzard Mountain Ski Area.

Continuing eastward, US-26 travels along the bank of Snake River and then enters Wyoming.

[2] In the Idaho Falls area, US-26 runs along local streets and county roads.

In 2022, the Idaho Transportation Board approved a study into relocating parts of the highway onto US-20 following the reconstruction of the I-15/US-20 interchange at the west end of the Rigby Freeway.

is a 2.981-mile-long (4.797 km) business route in and around the small town of Ririe northeast of Idaho Falls.