Otter National Forest was a brief early name for a federal forest reserve in Montana, established by the U.S. Forest Service on March 2, 1907. Covering 123,779 acres, it lasted only until July 1, 1908, when it was renamed Custer National Forest. The area later became the Ashland Ranger District within Custer National Forest, in what is now Powder River County and Rosebud County. Its significance lies in that short transition, linking an early forest reserve to the larger and longer-lasting national forest identity that continued afterward.