Plains is a town in Sanders County, Montana, United States. The population was 1,106 at the 2020 census. It was founded as Horse Plains and sometimes called Wild Horse Plains, as the local Native Americans would winter their horses here to feed on the grasses in the valley. The name was later shortened to Plains when the US Postal Service established a presence in 1905. The town was established in 1883 as a Northern Pacific Railroad station.