In the U.S. state of Montana, Interstate 15 (I-15) continues onward from Idaho for nearly 400 miles (640 km) through the cities of Butte, Helena, and Great Falls, intersecting with I-90, I-115, and I-315.

I-15 crosses into Montana from Idaho just south of Lima Reservoir over Monida Pass, at 6,870 feet (2,090 m), the highest elevation on the entire route of I-15.

In the town of Dillon, I-15 passes near Clark's Lookout State Park before beginning to parallel the Big Hole River.

The freeway provides access to Cascade and then continues through the plains through Ulm before turning northeast into the city of Great Falls.

Eventually, I-15 passes through the last town before Canada, Sweet Grass, and then crosses the international border, becoming Alberta Highway 4.