St. Clair County, Illinois

County seat: Belleville · Population 250,708 (2025) · 658 sq mi

St. Clair County is the 9th most populous of Illinois's 102 counties, with 250,708 people (2025 estimate). Its county seat is Belleville. It is part of the Southwest region, one of the 10 Illinois economic development regions (DCEO). The map shows its 30 cities and towns and the 7 neighboring counties.

Map of St. Clair County, Illinois with cities, towns and the county boundary
St. Clair County, Illinois with its cities and towns, unincorporated communities (CDPs) and neighboring counties. Boundaries: U.S. Census Bureau, 2025. Free to use with attribution (CC BY 4.0).
Download PDF (Letter) 157 KBPNG image 115 KBIllinois county map

St. Clair County facts

County seat
Belleville
Population (2025 estimate)
250,708 −2.7% since 2020
Land area
658 sq mi 28th largest of 102
Water area
16 sq mi
Population density
381 per sq mi
Established
1790
Same name
One of 4 St. Clair Counties in the US
Named after
Arthur St. Clair American soldier and politician (1737-1818)
Largest city
Belleville 40,237
Median household income
$73,854
Cities and towns
30 plus 5 CDPs
ZIP codes
34
Time zone
Central
FIPS code
17163

Cities and towns in St. Clair County

St. Clair County has 30 incorporated cities, towns and villages. Populations count the people living inside St. Clair County; cities that cross a county line also list the other counties.

NameTypePopulation 2025Also in
Belleville (county seat)City40,237
O'FallonCity32,656
East St. LouisCity17,682
Cahokia HeightsCity16,874
Fairview HeightsCity16,175
ShilohVillage14,604
SwanseaVillage14,356
MascoutahCity9,239
LebanonCity4,884
FreeburgVillage4,507
CaseyvilleVillage4,402
MillstadtVillage3,950
SmithtonVillage3,868
DupoVillage3,766Monroe County
CollinsvilleCity2,555Madison County
Washington ParkVillage2,461
Fairmont CityVillage2,132Madison County
New AthensVillage1,854
MarissaVillage1,724
BrooklynVillage626
St. LiboryVillage598
LenzburgVillage436
East CarondeletVillage370
SummerfieldVillage336
FayettevilleVillage285
New BadenVillage207Clinton County
ColumbiaCity191Monroe County
SaugetVillage129
HeckerVillage1Monroe County
MadisonCity1Madison County

Unincorporated communities (CDPs)

Census-designated places are communities without their own city government; the county provides their services.

CommunityPopulationAlso in
Scott AFB3,957
Paderborn20
Darmstadt18
Floraville0
Rentchler0

ZIP codes in St. Clair County

34 ZIP codes reach into St. Clair County. ZIP codes ignore county lines, so some also cover a neighboring county.

Population of St. Clair County, 2020–2025

St. Clair County shrank by 6,909 people (−2.7%) between April 2020 and July 2025: 1,953 more deaths than births and a net loss of 4,987 people from migration.

Year (July 1)Population
2020257,057
2021255,194
2022252,830
2023252,232
2024251,375
2025250,708

Neighboring counties

Printable St. Clair County map

The PDF is a vector file with real text: it prints sharp on Letter paper and you can search it for a town. The PNG is 2400 pixels wide for slides and documents. Both are free to use with a credit to WhichCounty.

Paste this code to embed the map on your site:

Sources

Back to the Illinois county map with all 102 counties. Found an error? Tell us.