Union Mountain is a 7,601-foot (2,317-metre) summit located in Flathead County of the U.S. state of Montana.

It is situated in the Great Bear Wilderness, on land managed by Flathead National Forest.

The nearest higher neighbor is Capitol Mountain, 1.9 miles (3.1 km) to the west-southwest.

Union Mountain is composed of sedimentary rock laid down during the Precambrian to Jurassic periods.

Formed in shallow seas, this sedimentary rock was initially uplifted beginning 170 million years ago when the Lewis Overthrust fault pushed an enormous slab of precambrian rocks 3 mi (4.8 km) thick, 50 miles (80 km) wide and 160 miles (260 km) long over younger rock of the cretaceous period.