Big Horn County is a county located in the U.S. state of Montana. As of the 2020 census, the population was 13,124. The county seat is Hardin. The county, like the river and the mountain range, is named after the bighorn sheep in the Rocky Mountains. The county was founded in 1913. It is located on the south line of the state.

Hardin is a city in and the county seat of Big Horn County, Montana, United States. The population was 3,818 at the 2020 census.