Plains is a town in Sanders County, Montana, United States. The population was estimated to be 1,071 in 2016. It was founded as Horse Plains and sometimes called Wild Horse Plains, as the local American Indians 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. Plains High School is a Class C school which serves local teenagers. Its students can participate in any of fourteen sports and extracurricular activities. This climatic region is typified by large seasonal temperature differences, with warm to hot (and often humid) summers and cold (sometimes severely cold) winters.