16 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| O'Fallon | 1 | 97,200 | 91,665 | 0.96% | 3,137 | 31 | City |
| St. Charles | 2 | 73,292 | 70,790 | 0.57% | 2,908 | 25 | City |
| St. Peters | 3 | 63,243 | 57,707 | 1.48% | 2,813 | 22 | City |
| Wentzville | 4 | 50,328 | 45,282 | 1.7% | 2,286 | 22 | City |
| Lake St. Louis | 5 | 20,345 | 16,789 | 3% | 2,382 | 9 | City |
| Dardenne Prairie | 6 | 15,220 | 12,780 | 2.75% | 2,853 | 5 | City |
| Cottleville | 7 | 6,716 | 5,701 | 2.58% | 1,419 | 5 | City |
| Weldon Spring | 8 | 5,289 | 5,333 | -0.13% | 692 | 8 | City |
| St. Paul | 9 | 4,057 | 3,061 | 4.27% | 563 | 7 | City |
| Foristell | 10 | 851 | 550 | 6.24% | 140 | 6 | City |
| New Melle | 11 | 546 | 549 | 0% | 305 | 2 | City |
| Josephville | 12 | 536 | 479 | 1.9% | 188 | 3 | Village |
| West Alton | 13 | 351 | 356 | -0.28% | 12 | 29 | City |
| Portage Des Sioux | 14 | 319 | 331 | -0.62% | 555 | 1 | City |
| Augusta | 15 | 266 | 271 | -0.37% | 258 | 1 | Town |
| Weldon Spring Heights | 16 | 91 | 94 | 0% | 948 | 0 | Town |
O'Fallon is the largest city in St. Charles County, with a population of 97,200.
Weldon Spring Heights is the smallest city in St. Charles County, with a population of 91.
There are 16 cities in St. Charles County, Missouri.