45 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Belleville city | 1 | 39,722 | 42,266 | -1.25% | 1,710 | 23.23 | City |
| Belleville | 2 | 39,699 | 42,266 | -1.28% | 1,709 | 23.23 | City |
| St. Clair township | 3 | 36,306 | 35,704 | 0.92% | 1,783 | 20.36 | Township |
| Caseyville township | 4 | 34,182 | 35,477 | -0.72% | 980 | 34.89 | Township |
| O'Fallon | 5 | 31,742 | 32,307 | -0.35% | 2,010 | 15.79 | City |
| O'Fallon township | 6 | 25,805 | 26,370 | -0.43% | 735 | 35.11 | Township |
| Centreville township | 7 | 18,865 | 20,132 | -1.26% | 675 | 27.93 | Township |
| East St. Louis township | 8 | 17,158 | 18,400 | -1.39% | 1,232 | 13.93 | Township |
| East St. Louis | 9 | 17,136 | 18,400 | -1.45% | 1,231 | 13.93 | City |
| Fairview Heights | 10 | 15,763 | 16,667 | -1.14% | 1,379 | 11.43 | City |
| Shiloh | 11 | 15,152 | 14,112 | 1.39% | 1,372 | 11.05 | Village |
| Swansea | 12 | 14,552 | 14,351 | 0.28% | 2,247 | 6.48 | Village |
| Shiloh Valley township | 13 | 12,960 | 12,822 | 0.29% | 408 | 31.77 | Township |
| Mascoutah township | 14 | 10,135 | 9,407 | 2.92% | 259 | 39.09 | Township |
| Mascoutah | 15 | 9,034 | 8,739 | 0.66% | 937 | 9.64 | City |
| Stookey township | 16 | 8,676 | 9,212 | -1.2% | 326 | 26.58 | Township |
| Canteen township | 17 | 7,610 | 8,001 | -1% | 448 | 17 | Township |
| Sugar Loaf township | 18 | 6,523 | 6,933 | -1.21% | 213 | 30.58 | Township |
| Millstadt township | 19 | 6,400 | 6,739 | -0.96% | 137 | 46.75 | Township |
| Freeburg township | 20 | 4,892 | 5,072 | -0.65% | 142 | 34.34 | Township |
| Lebanon township | 21 | 4,633 | 4,833 | -0.83% | 127 | 36.47 | Township |
| Lebanon | 22 | 4,499 | 4,679 | -0.79% | 1,669 | 2.69 | City |
| Freeburg | 23 | 4,447 | 4,582 | -0.6% | 649 | 6.85 | Village |
| Caseyville | 24 | 4,337 | 4,391 | -0.25% | 596 | 7.27 | Village |
| Scott AFB | 25 | 4,126 | 3,845 | 0.83% | 951 | 4.34 | CDP |
| Smithton township | 26 | 3,978 | 4,160 | -0.92% | 123 | 32.29 | Township |
| Millstadt | 27 | 3,861 | 4,065 | -1.05% | 1,101 | 3.51 | Village |
| Smithton | 28 | 3,831 | 4,003 | -0.88% | 851 | 4.50 | Village |
| Dupo | 29 | 3,717 | 3,976 | -1.38% | 581 | 6.40 | Village |
| Prairie Du Long township | 30 | 2,515 | 2,646 | -1.06% | 71 | 35.46 | Township |
| New Athens township | 31 | 2,373 | 2,526 | -1.25% | 70 | 34 | Township |
| Marissa township | 32 | 2,119 | 2,264 | -1.26% | 62 | 34.45 | Township |
| New Athens | 33 | 1,823 | 1,947 | -1.35% | 863 | 2.11 | Village |
| Marissa | 34 | 1,701 | 1,828 | -1.45% | 236 | 7.21 | Village |
| Fayetteville township | 35 | 1,443 | 1,525 | -1.23% | 34 | 42.96 | Township |
| Lenzburg township | 36 | 901 | 949 | -0.99% | 31 | 28.91 | Township |
| Engelmann township | 37 | 715 | 756 | -1.11% | 24 | 29.58 | Township |
| Stites township | 38 | 613 | 650 | -1.13% | 299 | 2.05 | Township |
| Brooklyn | 39 | 610 | 649 | -1.29% | 752 | 0.81 | Village |
| St. Libory | 40 | 597 | 629 | -1% | 615 | 0.97 | Village |
| Lenzburg | 41 | 440 | 467 | -1.12% | 381 | 1.16 | Village |
| East Carondelet | 42 | 365 | 390 | -1.35% | 310 | 1.18 | Village |
| Lovejoy township | 43 | 355 | 374 | -1.11% | 10 | 34.69 | Township |
| Summerfield | 44 | 324 | 343 | -1.22% | 632 | 0.51 | Village |
| Fayetteville | 45 | 281 | 293 | -0.71% | 949 | 0.30 | Village |
Belleville city is the largest city in St. Clair County, with a population of 39,722.
Fayetteville is the smallest city in St. Clair County.
There are 45 cities in St. Clair County, Illinois.