St. Clair is a city in St. Clair County in the eastern "Thumb" of the U.S. state of Michigan. The population was 5,485 at the 2010 census. The city is located on the St. Clair River near the southeast corner of St. Clair Township.

St. Clair County is a county located in the U.S. state of Michigan and bordering the west bank of the St. Clair River. As of the 2020 census, the population was 160,383. It is the 13th-most populous county in the state. The county seat is Port Huron, located at the north end of the St. Clair River at Lake Huron. The county was created September 10, 1820, and its government was organized in 1821. It is located northeast of Detroit. It is considered by the State of Michigan to be a part of The Thumb, a peninsula that is surrounded by Lake Huron in the east-central area of the state. This area is sometimes dubbed the Blue Water Area.