The highway continues southeasterly through woodlands, paralleling the St. Regis River, before meeting the Dena Mora (Lookout Pass) rest area 4.7 miles (7.6 km) east of the Idaho–Montana border.

[3] About 0.5 miles (0.80 km) from the rest stop is a diamond interchange, serving a small minor road into the St. Joe National Forest in Idaho.

The highway continues, winding through the woodlands until another diamond interchange, serving the small unincorporated community of Saltese.

Superior, the county seat, is intersected before the highway turns south through the Quartz Flats rest area, located 58.2 miles (93.7 km) from the Idaho border.

US 12 joins I-90 and MT 200 exits to the north of the highway outside of Missoula, paralleling the Blackfoot River as I-90 continues east, following and then intersecting S-210 in Clinton.

[3] Garrison is northeast of the highway as US 12 ends its concurrency, traveling north from I-90 toward the state capital of Helena.

I-90 turns south, paralleling former Northern Pacific Railway trackage,[7] before intersecting S-275 and the county seat, Deer Lodge.

Galen and S-273 are intersected, before the highway passes west of the Warm Springs State Wildlife Management Area.

[4][5] Fairmont Hot Springs Resort, accessible via S-441, is the first exit in Silver Bow County on I-90, as the highway starts to turn back toward the east.

An interchange with MT 2 serves Bert Mooney Airport before I-15/I-90 split just east of Butte, with I-15 continuing north over the Continental Divide over the 6,368-foot (1,941 m) high Elk Park Pass, which straddles the Silver Bow–Jefferson county border.

[8] I-90 intersects the Columbus rest area, 418.8 miles (674.0 km) from the Idaho border,[3] followed by Park City before exiting Stillwater County.

On the extreme eastern end of Billings, the three highways intersect the western terminus of I-94, which heads northeast toward North Dakota, while I-90 turns southeast into Big Horn County.

Immediately after entering Big Horn County, the three highways pass the seasonal Hardin rest area.

Continuing south, I-90/US 87 parallels S-451 and the Little Bighorn River, passing by Garryowen, Benteen, Lodge Grass, and Wyola before exiting the state of Montana and entering Sheridan County, Wyoming.