Gates of the Mountains Wilderness, Protected wilderness area in Helena-Lewis and Clark National Forest, Montana.

Gates of the Mountains Wilderness is a protected area in the Helena-Lewis and Clark National Forest where limestone cliffs rise straight up from the Missouri River. These vertical walls form a narrow passage through solid rock formations, with over 50 miles of trails winding through the landscape.

On July 19, 1805, Meriwether Lewis wrote that the party entered "the most remarkable clifts" rising 1,200 feet from the water and named them "the gates of the rocky mountains." Holter Dam, built in 1918, raised the Missouri's level in the canyon by about 14 feet. Congress designated the area a wilderness in 1964, and the 1949 Mann Gulch fire killed 13 firefighters there.

The name comes from Captain Lewis's journal entry of 19 July 1805, when he called the perpendicular cliffs "the gates of the rocky mountains". Mann Gulch, inside the wilderness, was the site of the 1949 fire that killed 13 firefighters, later told by Norman Maclean in Young Men and Fire. The Meriwether Picnic site near the canyon honours Lewis.

The wilderness lies north of Helena, reached via Gates of the Mountains Road off Interstate 15; the Meriwether Picnic site sits near the canyon entrance. No roads or buildings exist inside, and motorized or mechanized vehicles, bicycles included, are barred. Camping and fishing need a permit, and hunting is allowed in season.

The canyon is a superposed stream: the Missouri predates the uplift of the Madison Limestone and cut down through it as the rock rose. At the southern entrance, the Eldorado thrust fault places much older Greyson Shale directly over the younger limestone, a contact geologists come to see.