| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Belleville | 1 | 39,809 | 42,251 | -1.06% | 1,713 | 23 | City |
| O'Fallon | 2 | 32,600 | 32,410 | -0.17% | 2,064 | 16 | City |
| East St. Louis | 3 | 17,559 | 18,431 | -0.7% | 1,261 | 14 | City |
| Cahokia Heights | 4 | 16,678 | 17,820 | -1.16% | 1,061 | 16 | City |
| Fairview Heights | 5 | 16,038 | 16,694 | -0.85% | 1,403 | 11 | City |
| Shiloh | 6 | 14,743 | 14,098 | 0.95% | 1,165 | 13 | Village |
| Swansea | 7 | 14,382 | 14,336 | 0.18% | 2,220 | 6 | Village |
| Mascoutah | 8 | 9,322 | 8,779 | 0.9% | 967 | 10 | City |
| Lebanon | 9 | 4,879 | 4,760 | -0.1% | 1,810 | 3 | City |
| Freeburg | 10 | 4,487 | 4,599 | -0.44% | 655 | 7 | Village |
| Caseyville | 11 | 4,405 | 4,387 | 0.07% | 606 | 7 | Village |
| Millstadt | 12 | 3,921 | 4,083 | -0.73% | 1,118 | 4 | Village |
| Smithton | 13 | 3,840 | 4,003 | -0.72% | 853 | 5 | Village |
| Dupo | 14 | 3,721 | 3,981 | -1.19% | 581 | 6 | Village |
| New Athens | 15 | 1,834 | 1,952 | -1.08% | 868 | 2 | Village |
| Marissa | 16 | 1,702 | 1,826 | -1.28% | 236 | 7 | Village |
| Brooklyn | 17 | 620 | 653 | -0.96% | 764 | 1 | Village |
| St. Libory | 18 | 592 | 627 | -1% | 610 | 1 | Village |
| Lenzburg | 19 | 431 | 457 | -1.15% | 373 | 1 | Village |
| East Carondelet | 20 | 366 | 389 | -1.08% | 311 | 1 | Village |
| Summerfield | 21 | 332 | 353 | -1.19% | 647 | 1 | Village |
| Fayetteville | 22 | 283 | 293 | -0.7% | 956 | 0 | Village |
Belleville is the largest city in St. Clair County, with a population of 39,809.
Fayetteville is the smallest city in St. Clair County, with a population of 283.
There are 22 cities in St. Clair County, Illinois.