Petroleum County is a county in the U.S. state of Montana. As of the 2020 census, the population was 496, making it the least populous county in Montana and the eighth-least populous in the United States. Its county seat is Winnett. The county's area was partitioned from Fergus County in 1925 to become the last of Montana's 56 counties to be organized.Soils Industry, pp. 1-2
