St. Clair County is a county located in the central portion of the U.S. state of Alabama. As of the 2020 census, the population was 91,103. It has two county seats: Ashville and Pell City. It is one of two counties in Alabama, and one of 33 in the United States, with more than one county seat. Its name is in honor of General Arthur St. Clair, an officer in the French and Indian War. St. Clair County is included in the Birmingham, Alabama Metropolitan Statistical Area.

A county seat is an administrative center, seat of government, or capital city of a county or civil parish. The term is in use in five countries: Canada, China, Hungary, Romania, and the United States. An equivalent term, shire town, is used in the U.S. state of Vermont and in several other English-speaking jurisdictions. County towns usually serve a similar function in Ireland, Jamaica, and the United Kingdom.