45 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Belleville | 1 | 40,244 | 42,278 | -0.78% | 1,732 | 23 | City |
| Belleville | 2 | 40,130 | 42,278 | -1.06% | 1,727 | 23 | City |
| St. Clair | 3 | 35,370 | 35,691 | -0.53% | 1,737 | 20 | Township |
| Caseyville | 4 | 34,602 | 35,478 | -0.28% | 992 | 35 | Township |
| O'Fallon | 5 | 32,108 | 32,413 | -0.19% | 2,033 | 16 | City |
| O'Fallon | 6 | 26,637 | 26,937 | -0.11% | 759 | 35 | Township |
| Centreville | 7 | 19,095 | 20,133 | -0.82% | 684 | 28 | Township |
| East St. Louis | 8 | 17,907 | 18,401 | 0.56% | 1,286 | 14 | Township |
| East St. Louis | 9 | 17,659 | 18,401 | -0.84% | 1,268 | 14 | City |
| Fairview Heights | 10 | 15,977 | 16,666 | -0.86% | 1,398 | 11 | City |
| Shiloh | 11 | 14,907 | 14,139 | 1.04% | 1,349 | 11 | Village |
| Swansea | 12 | 14,468 | 14,353 | 0.16% | 2,234 | 6 | Village |
| Shiloh Valley | 13 | 12,796 | 12,825 | -0.48% | 403 | 32 | Township |
| Mascoutah | 14 | 9,886 | 9,453 | 1.28% | 253 | 39 | Township |
| Mascoutah | 15 | 9,201 | 8,785 | 0.91% | 954 | 10 | City |
| Stookey | 16 | 8,798 | 9,210 | -0.69% | 331 | 27 | Township |
| Canteen | 17 | 7,726 | 8,006 | -0.52% | 454 | 17 | Township |
| Sugar Loaf | 18 | 6,592 | 6,930 | -0.77% | 216 | 31 | Township |
| Millstadt | 19 | 6,503 | 6,744 | -0.38% | 139 | 47 | Township |
| Freeburg | 20 | 4,923 | 5,070 | -0.49% | 143 | 34 | Township |
| Lebanon | 21 | 4,598 | 4,747 | -0.65% | 1,706 | 3 | City |
| Freeburg | 22 | 4,496 | 4,603 | -0.49% | 656 | 7 | Village |
| Caseyville | 23 | 4,395 | 4,392 | 0% | 604 | 7 | Village |
| Lebanon | 24 | 4,276 | 4,436 | -0.58% | 117 | 36 | Township |
| Scott AFB | 25 | 4,126 | 3,845 | 0.83% | 951 | 4 | CDP |
| Smithton | 26 | 4,019 | 4,159 | -0.5% | 124 | 32 | Township |
| Millstadt | 27 | 3,928 | 4,073 | -0.73% | 1,120 | 4 | Village |
| Smithton | 28 | 3,859 | 4,003 | -0.75% | 857 | 5 | Village |
| Dupo | 29 | 3,752 | 3,976 | -1.19% | 586 | 6 | Village |
| Prairie Du Long | 30 | 2,555 | 2,649 | -0.47% | 72 | 35 | Township |
| New Athens | 31 | 2,408 | 2,526 | -0.7% | 71 | 34 | Township |
| Marissa | 32 | 2,149 | 2,264 | -0.83% | 62 | 34 | Township |
| New Athens | 33 | 1,850 | 1,948 | -1.07% | 876 | 2 | Village |
| Marissa | 34 | 1,721 | 1,828 | -1.26% | 239 | 7 | Village |
| Fayetteville | 35 | 1,468 | 1,527 | -0.68% | 34 | 43 | Township |
| Lenzburg | 36 | 904 | 947 | -0.66% | 31 | 29 | Township |
| Engelmann | 37 | 728 | 755 | -0.27% | 25 | 30 | Township |
| Stites | 38 | 618 | 651 | -0.96% | 302 | 2 | Township |
| Brooklyn | 39 | 616 | 650 | -1.12% | 760 | 1 | Village |
| St. Libory | 40 | 602 | 629 | -0.99% | 621 | 1 | Village |
| Lenzburg | 41 | 443 | 465 | -1.12% | 384 | 1 | Village |
| East Carondelet | 42 | 369 | 389 | -1.07% | 314 | 1 | Village |
| Lovejoy | 43 | 357 | 373 | -0.56% | 10 | 35 | Township |
| Summerfield | 44 | 328 | 343 | -0.91% | 639 | 1 | Village |
| Fayetteville | 45 | 284 | 294 | -0.7% | 959 | 0 | Village |
Belleville is the largest city in St. Clair County, with a population of 40,244.
Fayetteville is the smallest city in St. Clair County, with a population of 284.
There are 45 cities in St. Clair County, Illinois.