I-15 crosses into Montana from Idaho just south of
Lima Reservoir over
Monida Pass, at , the highest elevation on the entire route of I-15. The route continues northwest through farmland and desert. The freeway turns north at
Clark Canyon Reservoir, before turning northeast. In the town of
Dillon, I-15 passes near
Clark's Lookout State Park before beginning to parallel the
Big Hole River. The freeway then turns away from the river, continuing northeast along Divide Creek through the desert, before splitting away from the creek and heading north. I-15 eventually merges into
I-90 at a
trumpet interchange. I-15 and I-90 run concurrently through the city of
Butte, coming first to an interchange with
I-115, signed as Business Loop 15 and Business Loop 90. I-15 and I-90 pass through a green area in the middle of the city, before I-15 splits off to the northeast on the eastern end of the town. The freeway climbs along the eastern edge of the Continental Pit mine, and then over
continental divide at
Elk Park Pass. I-15 travels through the forest and plains of Elk Park before winding along Bison Creek and later the
Boulder River. I-15 passes through the town of
Basin before leaving the mountains and providing access to
Boulder. Paralleling Beavertown Creek, the road winds through the hills before passing through
Jefferson City. The freeway continues into
Helena, where I-15 intersects
US 12 and runs concurrently with
US 287 through the outskirts of the city before continuing north through the plains. I-15 and US 287 continue along Little Prickly Pear Creek before passing through the town of
Wolf Creek after several miles. Northeast of there, US 287 continues north, while I-15 runs close to the
Missouri River. The freeway provides access to
Cascade and then continues through the plains through
Ulm before turning northeast into the city of
Great Falls.
I-315 and
US 89 intersect with I-15 at a trumpet interchange, and US 89 continues north and then northwest along I-15 for a few miles before continuing to the west. I-15 continues northwest through farmland before passing near
Dutton and
Brady, and the city of
Conrad.
US 2 intersects I-15 in the town of
Shelby. The freeway passes through the town of
Sunburst, then passes through a final stretch of farmland. Eventually, I-15 passes through the last town before Canada,
Sweet Grass, and then crosses the international border, becoming
Alberta Highway 4. ==History==