22 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Belleville | 1 | 39,701 | 42,278 | -1.07% | 1,709 | 23 | City |
| O'Fallon | 2 | 32,047 | 32,413 | -0.19% | 2,029 | 16 | City |
| East St. Louis | 3 | 17,511 | 18,401 | -0.84% | 1,258 | 14 | City |
| Cahokia Heights | 4 | 16,653 | 17,849 | -1.18% | 1,060 | 16 | City |
| Fairview Heights | 5 | 15,839 | 16,666 | -0.86% | 1,386 | 11 | City |
| Shiloh | 6 | 15,061 | 14,139 | 1.03% | 1,190 | 13 | Village |
| Swansea | 7 | 14,491 | 14,353 | 0.16% | 2,237 | 6 | Village |
| Mascoutah | 8 | 9,284 | 8,785 | 0.9% | 963 | 10 | City |
| Lebanon | 9 | 4,568 | 4,747 | -0.65% | 1,695 | 3 | City |
| Freeburg | 10 | 4,475 | 4,603 | -0.47% | 653 | 7 | Village |
| Caseyville | 11 | 4,396 | 4,392 | 0.02% | 605 | 7 | Village |
| Millstadt | 12 | 3,899 | 4,073 | -0.74% | 1,112 | 4 | Village |
| Smithton | 13 | 3,830 | 4,003 | -0.75% | 851 | 5 | Village |
| Dupo | 14 | 3,707 | 3,976 | -1.2% | 579 | 6 | Village |
| New Athens | 15 | 1,831 | 1,948 | -1.03% | 867 | 2 | Village |
| Marissa | 16 | 1,700 | 1,828 | -1.22% | 236 | 7 | Village |
| Brooklyn | 17 | 609 | 650 | -1.14% | 751 | 1 | Village |
| St. Libory | 18 | 597 | 629 | -0.83% | 615 | 1 | Village |
| Lenzburg | 19 | 439 | 465 | -0.9% | 380 | 1 | Village |
| East Carondelet | 20 | 365 | 389 | -1.08% | 310 | 1 | Village |
| Summerfield | 21 | 325 | 343 | -0.91% | 634 | 1 | Village |
| Fayetteville | 22 | 282 | 294 | -0.7% | 953 | 0 | Village |
Belleville is the largest city in St. Clair County, with a population of 39,701.
Fayetteville is the smallest city in St. Clair County, with a population of 282.
There are 22 cities in St. Clair County, Illinois.