St. Clair County
Also known as St. Clair County, Illinois, St. Clair County, IL, Saint Clair County
county in Illinois, United States
Key facts
- Country
- United States
- State
- Illinois
- Named after
- Arthur St. Clair
- Seat
- Belleville
- Largest city
- Belleville
- Total
- 674 sq mi (1,750 km )
- Land
- 658 sq mi (1,700 km )
- Water
- 16 sq mi (41 km ) 2.4%
- Estimate 2025
- 250,708
- Density
- 391/sq mi (151/km )
- Time zone
- UTC−6 ( Central )
- Summer dst
- UTC−5 ( CDT )
- Congressional districts
Article
House in Lebanon, Il historic district Mermaid House Hotel, Lebanon, Illinois
St. Clair County is the ninth most populous county in Illinois. Located directly east of St. Louis, the county is part of the Metro East region of the Greater St. Louis metropolitan area in southern Illinois. As of the 2020 United States census, St. Clair County had a population of 257,400, making it the second most populous county in Illinois outside the Northern Third. Belleville is the county seat and largest city.