Big Baldy Mountain, Mountain summit in Judith Basin County, Montana, United States.

Big Baldy Mountain is the highest peak of the Little Belt Mountains, in Judith Basin County, Montana, reaching 9,181 feet (2,798 m). It lies just over 10 km east of U.S. Route 89, within Lewis and Clark National Forest. The summit is bare of vegetation, which likely gave the mountain its name; locally it is also called Mount Baldy.

Big Baldy Mountain lies in the Little Belt Mountains, not the Rocky Mountains proper. Area residents and some scientists hold that the peak formed from magma that never reached the surface: as the Rocky Mountains were building, magma intruded close to the surface beneath the Little Belts, and volcanic gases escaped, leaving sunken craters. Satellite views show craters, summit lakes and stains resembling an old lava trail.

Locals call the peak Mount Baldy, and the name is tied to its bare summit, where no vegetation grows. Some residents and scientists hold that the mountain and nearby ranges formed from magma that never reached the surface, and satellite images show craters and summit lakes read as signs of that past.

Reaching the summit requires driving on forest service roads followed by a hiking trek that gains considerable elevation over several hours. Weather conditions at altitude can change rapidly, so proper preparation and sturdy gear are important for the outing.

Vegetation stops well below the summit, creating the characteristic bald appearance that inspired the mountain's name. This natural tree line boundary resulted from the peak's altitude and exposure to wind and harsh alpine conditions.