Castle Reef is an 8,330-foot (2,539-metre) mountain summit located in Teton County of the U.S. state of Montana.

It is situated 26 miles west-southwest of Choteau, in Lewis and Clark National Forest.

Topographic relief is significant as the east aspect rises nearly 4,000 feet (1,200 meters) above the prairie.

Castle Reef is composed of sedimentary rock laid down during the Precambrian to Jurassic periods.

[4] The Lewis Overthrust extends over 280 miles (450 km) from Mount Kidd in Alberta, south to Steamboat Mountain which is located 25 miles south of Castle Reef, which places Castle Reef within the southern extent of the Lewis Overthrust.