Potosi is a city in Washington County, Missouri, United States. Potosi is seventy-two miles southwest of St. Louis. The population was 2,660 at the 2010 census. It is the county seat of Washington County. The city was founded sometime between 1760 and 1780 as "Mine à Breton" or Mine au Breton, and later renamed by Moses Austin for the Bolivian silver-mining city of Potosí.