44 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Belleville | 1 | 39,928 | 42,278 | -0.79% | 1,719 | 23 | City |
| Belleville | 2 | 39,701 | 42,278 | -1.07% | 1,709 | 23 | City |
| St. Clair | 3 | 35,180 | 35,691 | -0.54% | 1,728 | 20 | Township |
| Caseyville | 4 | 34,505 | 35,478 | -0.28% | 989 | 35 | Township |
| O'Fallon | 5 | 32,047 | 32,413 | -0.19% | 2,029 | 16 | City |
| O'Fallon | 6 | 26,608 | 26,937 | -0.11% | 758 | 35 | Township |
| Centreville | 7 | 18,937 | 20,133 | -0.83% | 678 | 28 | Township |
| East St. Louis | 8 | 18,006 | 18,401 | 0.55% | 1,293 | 14 | Township |
| East St. Louis | 9 | 17,511 | 18,401 | -0.84% | 1,258 | 14 | City |
| Fairview Heights | 10 | 15,839 | 16,666 | -0.86% | 1,386 | 11 | City |
| Shiloh | 11 | 15,061 | 14,139 | 1.03% | 1,190 | 13 | Village |
| Swansea | 12 | 14,491 | 14,353 | 0.16% | 2,237 | 6 | Village |
| Shiloh Valley | 13 | 12,734 | 12,825 | -0.48% | 401 | 32 | Township |
| Mascoutah | 14 | 10,011 | 9,453 | 1.26% | 256 | 39 | Township |
| Mascoutah | 15 | 9,284 | 8,785 | 0.9% | 963 | 10 | City |
| Stookey | 16 | 8,737 | 9,210 | -0.69% | 329 | 27 | Township |
| Canteen | 17 | 7,686 | 8,006 | -0.52% | 452 | 17 | Township |
| Sugar Loaf | 18 | 6,541 | 6,930 | -0.77% | 214 | 31 | Township |
| Millstadt | 19 | 6,478 | 6,744 | -0.38% | 139 | 47 | Township |
| Freeburg | 20 | 4,899 | 5,070 | -0.49% | 143 | 34 | Township |
| Lebanon | 21 | 4,568 | 4,747 | -0.65% | 1,695 | 3 | City |
| Freeburg | 22 | 4,475 | 4,603 | -0.47% | 653 | 7 | Village |
| Caseyville | 23 | 4,396 | 4,392 | 0.02% | 605 | 7 | Village |
| Lebanon | 24 | 4,251 | 4,436 | -0.58% | 117 | 36 | Township |
| Smithton | 25 | 3,999 | 4,159 | -0.5% | 124 | 32 | Township |
| Millstadt | 26 | 3,899 | 4,073 | -0.74% | 1,112 | 4 | Village |
| Smithton | 27 | 3,830 | 4,003 | -0.75% | 851 | 5 | Village |
| Dupo | 28 | 3,707 | 3,976 | -1.2% | 579 | 6 | Village |
| Prairie Du Long | 29 | 2,543 | 2,649 | -0.47% | 72 | 35 | Township |
| New Athens | 30 | 2,391 | 2,526 | -0.71% | 70 | 34 | Township |
| Marissa | 31 | 2,131 | 2,264 | -0.84% | 62 | 34 | Township |
| New Athens | 32 | 1,831 | 1,948 | -1.03% | 867 | 2 | Village |
| Marissa | 33 | 1,700 | 1,828 | -1.22% | 236 | 7 | Village |
| Fayetteville | 34 | 1,458 | 1,527 | -0.68% | 34 | 43 | Township |
| Lenzburg | 35 | 898 | 947 | -0.66% | 31 | 29 | Township |
| Engelmann | 36 | 726 | 755 | -0.27% | 25 | 30 | Township |
| Stites | 37 | 612 | 651 | -0.97% | 299 | 2 | Township |
| Brooklyn | 38 | 609 | 650 | -1.14% | 751 | 1 | Village |
| St. Libory | 39 | 597 | 629 | -0.83% | 615 | 1 | Village |
| Lenzburg | 40 | 439 | 465 | -0.9% | 380 | 1 | Village |
| East Carondelet | 41 | 365 | 389 | -1.08% | 310 | 1 | Village |
| Lovejoy | 42 | 355 | 373 | -0.56% | 10 | 35 | Township |
| Summerfield | 43 | 325 | 343 | -0.91% | 634 | 1 | Village |
| Fayetteville | 44 | 282 | 294 | -0.7% | 953 | 0 | Village |
Belleville is the largest city in St. Clair County, with a population of 39,928.
Fayetteville is the smallest city in St. Clair County, with a population of 282.
There are 44 cities in St. Clair County, Illinois.