Petroleum County is a county in the state of Montana, established in 1925. As of the 2010 United States Census, the population was 494, making it the least populous county in Montana and the seventh-least populous in the United States.
Its county seat is Winnett. The county’s area was partitioned from Fergus County to become one of the last of Montana’s 56 counties to be organized.
Walter John Winnett, a rancher who had been adopted into the Sioux tribe, started a ranch in Montana Territory in 1879. His ranch house (built in 1900) became a gathering place for the area.
In 1910, he built a store and petitioned for a post office; thus Winnett became an official town.