Note: As defined by the US Census Bureau, a city's official boundaries often extend miles beyond the city itself and may even cross county lines. As such, several thousand US cities officially exist in two or more counties (and therefore appear on multiple counties' lists of cities.)
City | Rank | 2024 Pop. | 2020 Pop. | Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Belleville | 1 | 40,212 | 42,266 | -1.26% | 1,731 | 23.2 | City |
| O'Fallon | 2 | 31,855 | 32,307 | -0.35% | 2,017 | 15.8 | City |
| East St. Louis | 3 | 17,389 | 18,400 | -1.43% | 1,249 | 13.9 | City |
| Fairview Heights | 4 | 15,944 | 16,667 | -1.12% | 1,395 | 11.4 | City |
| Shiloh | 5 | 14,944 | 14,112 | 1.41% | 1,353 | 11.0 | Village |
| Swansea | 6 | 14,512 | 14,351 | 0.28% | 2,241 | 6.5 | Village |
| Mascoutah | 7 | 8,975 | 8,739 | 0.66% | 931 | 9.6 | City |
| Lebanon | 8 | 4,535 | 4,679 | -0.79% | 1,683 | 2.7 | City |
| Freeburg | 9 | 4,474 | 4,582 | -0.6% | 653 | 6.8 | Village |
| Caseyville | 10 | 4,348 | 4,391 | -0.25% | 598 | 7.3 | Village |
| Scott AFB | 11 | 4,092 | 3,845 | 0.86% | 944 | 4.3 | CDP |
| Millstadt | 12 | 3,902 | 4,065 | -1.04% | 1,113 | 3.5 | Village |
| Smithton | 13 | 3,865 | 4,003 | -0.9% | 858 | 4.5 | Village |
| Dupo | 14 | 3,769 | 3,976 | -1.36% | 589 | 6.4 | Village |
| New Athens | 15 | 1,848 | 1,947 | -1.33% | 875 | 2.1 | Village |
| Marissa | 16 | 1,726 | 1,828 | -1.48% | 239 | 7.2 | Village |
| Brooklyn | 17 | 618 | 649 | -1.28% | 762 | 0.8 | Village |
| St. Libory | 18 | 603 | 629 | -1.15% | 622 | 1.0 | Village |
| Lenzburg | 19 | 445 | 467 | -1.33% | 385 | 1.2 | Village |
| East Carondelet | 20 | 370 | 390 | -1.33% | 314 | 1.2 | Village |
| Summerfield | 21 | 328 | 343 | -1.21% | 639 | 0.5 | Village |
| Fayetteville | 22 | 283 | 293 | -1.05% | 956 | 0.3 | Village |
Note: City markers shown outside the county boundary mean that a part of the city extends into this county, but is primarily located in an adjacent county.
Belleville is the largest city in St. Clair County.
Fayetteville is the smallest city in St. Clair County.
There are 22 cities in St. Clair County, Illinois.