18 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| O'Fallon | 1 | 95,709 | 91,621 | 0.86% | 3,135 | 30.52 | City |
| St. Charles | 2 | 72,480 | 70,780 | 0.47% | 2,876 | 25.20 | City |
| St. Peters | 3 | 60,549 | 57,708 | 0.95% | 2,694 | 22.48 | City |
| Wentzville | 4 | 49,187 | 44,962 | 1.75% | 2,303 | 21.36 | City |
| Lake St. Louis | 5 | 19,910 | 16,787 | 3.24% | 2,331 | 8.54 | City |
| Dardenne Prairie | 6 | 14,489 | 12,774 | 2.42% | 2,716 | 5.33 | City |
| Cottleville | 7 | 6,274 | 5,696 | 1.88% | 1,326 | 4.73 | City |
| Weldon Spring | 8 | 5,217 | 5,332 | -0.44% | 682 | 7.65 | City |
| St. Paul | 9 | 3,996 | 3,060 | 4.91% | 555 | 7.21 | City |
| Foristell | 10 | 659 | 551 | 3.45% | 108 | 6.08 | City |
| Josephville | 11 | 561 | 516 | 1.63% | 197 | 2.85 | Village |
| New Melle | 12 | 534 | 544 | -0.37% | 317 | 1.69 | City |
| West Alton | 13 | 346 | 356 | -0.57% | 12 | 28.81 | City |
| Portage Des Sioux | 14 | 319 | 333 | -0.93% | 555 | 0.57 | City |
| Augusta | 15 | 260 | 269 | -0.76% | 295 | 0.88 | Town |
| Weldon Spring Heights | 16 | 90 | 94 | -1.1% | 938 | 0.10 | Town |
| Defiance | 17 | 62 | 68 | -10.14% | 61 | 1.01 | CDP |
| Three Creeks | 18 | 6 | 6 | 0% | 1 | 4.78 | Village |
O'Fallon is the largest city in St. Charles County, with a population of 95,709.
Three Creeks is the smallest city in St. Charles County.
There are 18 cities in St. Charles County, Missouri.