There are 42 cities in St. Clair County, Illinois.
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 | State | 2024 Population | 2020 Census | Annual Change | Density (mi²) | Area (mi²) |
|---|---|---|---|---|---|---|---|
| Belleville | 1 | Illinois | 40,329 | 42,261 | -1.18% | 1,736 | 23.23 |
| O'Fallon | 2 | Illinois | 31,977 | 32,303 | -0.25% | 2,025 | 15.79 |
| Granite City | 3 | Illinois | 26,728 | 27,514 | -0.73% | 1,406 | 19.02 |
| Collinsville | 4 | Illinois | 23,622 | 24,322 | -0.73% | 1,550 | 15.24 |
| East St. Louis | 5 | Illinois | 17,435 | 18,403 | -1.37% | 1,252 | 13.93 |
| Fairview Heights | 6 | Illinois | 15,983 | 16,665 | -1.05% | 1,398 | 11.43 |
| Shiloh | 7 | Illinois | 14,882 | 14,110 | 1.31% | 1,347 | 11.05 |
| Swansea | 8 | Illinois | 13,871 | 14,353 | -0.86% | 2,142 | 6.48 |
| Troy | 9 | Illinois | 11,336 | 10,926 | 0.92% | 1,991 | 5.69 |
| Waterloo | 10 | Illinois | 11,255 | 11,005 | 0.56% | 1,367 | 8.23 |
| Columbia | 11 | Illinois | 10,909 | 11,031 | -0.27% | 1,034 | 10.55 |
| Mascoutah | 12 | Illinois | 8,530 | 8,738 | -0.61% | 885 | 9.64 |
| Maryville | 13 | Illinois | 8,358 | 8,230 | 0.38% | 1,448 | 5.77 |
| Pontoon Beach | 14 | Illinois | 5,793 | 5,863 | -0.29% | 581 | 9.97 |
| Lebanon | 15 | Illinois | 4,546 | 4,682 | -0.74% | 1,687 | 2.69 |
| Freeburg | 16 | Illinois | 4,473 | 4,585 | -0.62% | 653 | 6.85 |
| Caseyville | 17 | Illinois | 4,391 | 4,391 | 0% | 604 | 7.27 |
| Scott AFB | 18 | Illinois | 4,201 | 3,845 | 2.16% | 969 | 4.34 |
| Millstadt | 19 | Illinois | 3,897 | 4,065 | -1.07% | 1,112 | 3.51 |
| Smithton | 20 | Illinois | 3,890 | 4,002 | -0.71% | 864 | 4.50 |
| Dupo | 21 | Illinois | 3,774 | 3,976 | -1.31% | 589 | 6.40 |
| New Baden | 22 | Illinois | 3,352 | 3,420 | -0.51% | 1,837 | 1.82 |
| Madison | 23 | Illinois | 3,046 | 3,138 | -0.75% | 215 | 14.14 |
| Trenton | 24 | Illinois | 2,629 | 2,683 | -0.49% | 1,396 | 1.88 |
| Washington Park | 25 | Illinois | 2,471 | 2,573 | -1% | 968 | 2.55 |
| Fairmont City | 26 | Illinois | 2,151 | 2,255 | -1.19% | 347 | 6.21 |
| New Athens | 27 | Illinois | 1,855 | 1,947 | -1.23% | 878 | 2.11 |
| Marissa | 28 | Illinois | 1,732 | 1,830 | -1.37% | 240 | 7.21 |
| Venice | 29 | Illinois | 1,446 | 1,490 | -0.76% | 782 | 1.85 |
| Valmeyer | 30 | Illinois | 1,209 | 1,229 | -0.41% | 353 | 3.42 |
| Albers | 31 | Illinois | 1,099 | 1,123 | -0.54% | 1,108 | 0.99 |
| Brooklyn | 32 | Illinois | 620 | 650 | -1.12% | 764 | 0.81 |
| St. Libory | 33 | Illinois | 606 | 628 | -0.82% | 625 | 0.97 |
| Lenzburg | 34 | Illinois | 446 | 466 | -1.11% | 386 | 1.16 |
| Hecker | 35 | Illinois | 418 | 428 | -0.48% | 1,566 | 0.27 |
| East Carondelet | 36 | Illinois | 369 | 389 | -1.34% | 314 | 1.18 |
| Summerfield | 37 | Illinois | 331 | 343 | -0.9% | 645 | 0.51 |
| Fayetteville | 38 | Illinois | 290 | 296 | -0.34% | 980 | 0.30 |
| Sauget | 39 | Illinois | 130 | 138 | -1.52% | 28 | 4.59 |
| Floraville | 40 | Illinois | 14 | 30 | -22.22% | 177 | 0.08 |
| Paderborn | 41 | Illinois | 0 | 65 | 0% | 0 | 0.15 |
| Rentchler | 42 | Illinois | 0 | 4 | 0% | 0 | 0.31 |
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.
Rentchler is the smallest city in St. Clair County.
There are 42 cities in St. Clair County, Illinois.