St. Regis or Saint Regis is an unincorporated community and census-designated place in Mineral County, Montana. The population was 313 at the time of the 2020 census. Pierre-Jean De Smet travelled extensively in this area of Montana. He named the river, after which the town takes its name, after a fellow Jesuit priest. The post office opened in 1896. The Northern Pacific Railway built a railroad through the town in the early 1900's, boosting the economy.