Fallon County is a county located in the U.S. state of Montana. As of the 2020 census, the population was 3,049. Its county seat is Baker. The county was created in 1913 from a portion of Custer County. It is named for Benjamin O'Fallon, a nephew of Captain William Clark and an Indian agent for the upper Missouri region from 1823 to 1827.

Baker is a city in and the county seat of Fallon County, Montana, United States. The population was 1,802 at the 2020 census.