Potosi is a city in Washington County, Missouri, United States. Potosi is seventy-two miles southwest of St. Louis. The population was 2,538 as of the 2020 census. It is the county seat of Washington County. Located in the Lead Belt, the city was founded in 1763 by French colonists as Mine à Breton or Mine au Breton. After the United States acquired this area, American Moses Austin renamed the community for the Bolivian silver-mining city of Potosí. He was involved in lead mining.