Gallatin National Forest, National forest in Montana, United States

Gallatin National Forest covers about 2,129,194 acres in southwestern Montana and borders Yellowstone National Park on the northwest, forming part of the Greater Yellowstone Ecosystem. Six mountain ranges lie within it, including the Gallatin, Madison, Bridger, Crazy, Absaroka and Beartooth. Granite Peak, Montana's highest point at 12,799 ft (3,901 m), rises in the Beartooth Range, and the forest also holds the Absaroka-Beartooth and Lee Metcalf wilderness areas.

Gallatin National Forest was established on February 10, 1899, with an area of 2,129,194 acres. It borders Yellowstone National Park and forms part of the Greater Yellowstone Ecosystem. Since 2014 it has been managed together with Custer National Forest as the Custer Gallatin National Forest, headquartered in Bozeman, Montana, with ranger district offices in West Yellowstone, Livingston, Gardiner and Bozeman.

The forest's name honors Albert Gallatin (1761–1849), U.S. Secretary of the Treasury and a scholar of Native American languages. Within the forest are Native American burial grounds, pictographs and petroglyphs; for the Plains Indians the forest provided shelter and a stable food supply. Members of the Lewis and Clark Expedition are generally considered the first white Americans to visit the region.

The forest can be accessed from several directions, with numerous marked trails that offer different difficulty levels and lengths. Visitors should prepare for changing weather conditions, especially at higher elevations where temperature shifts and snow are common.

Grasshopper Glacier in the Beartooth Range holds millions of grasshoppers that died about 300 years ago, frozen inside the ice. The forest also contains Native American burial grounds, pictographs and petroglyphs. Granite Peak, Montana's highest point at 12,799 ft (3,901 m), lies partly within the forest.