Where Is St. Clair County? All 4 St. Clair Counties on a Map

Where is St. Clair County?

4 St. Clair Counties in 4 states · 519,376 people (2025)

There are 4 counties named St. Clair County in the United States, in Alabama, Illinois, Michigan and Missouri. The most populous is St. Clair County, Illinois (county seat: Belleville), with 250,708 people in 2025. 3 of them are named after Arthur St. Clair, American soldier and politician (1737-1818).

All 4 St. Clair Counties

Sorted by state. Each link opens the county's map.

State	County seat	Population 2025	Land area (sq mi)	Established	Named after
St. Clair County, Alabama	Ashville, Pell City	98,206	632	1818	Arthur St. Clair
St. Clair County, Illinois	Belleville	250,708	658	1790	Arthur St. Clair
St. Clair County, Michigan	Port Huron	160,486	722	1820	Lake Saint Clair, St. Clair River
St. Clair County, Missouri	Osceola	9,976	675	1841	Arthur St. Clair

Questions about St. Clair Counties

How many St. Clair Counties are there?

There are 4 St. Clair Counties in the United States, in Alabama, Illinois, Michigan and Missouri.

Which St. Clair County is the largest?

By population, St. Clair County, Illinois (250,708 people in 2025). By land area, St. Clair County, Michigan (722 sq mi).

Which St. Clair County is the oldest?

St. Clair County, Illinois, established in 1790. The newest is St. Clair County, Missouri (1841).

Why are there so many St. Clair Counties?

3 of the 4 are named after Arthur St. Clair, American soldier and politician (1737-1818). Many counties took their names from the same founders, presidents and generals as new states were organized.

How do I find out which St. Clair County an address is in?

Open the county map of the state: it has a search box for cities, towns and ZIP codes. On a phone, the "what county am I in" tool on our sister site uses your location.

What county am I in? →