Otter is an unincorporated community in southwestern Powder River County, Montana, United States, in the Custer National Forest. It lies along local roads southwest of the town of Broadus, the county seat of Powder River County. Its elevation is 3,484 feet (1,062 m). Although Otter is unincorporated, it has a post office, with the ZIP code of 59062.
According to the Köppen Climate Classification system, Otter has a semi-arid climate, abbreviated "BSk" on climate maps.