| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| O'Fallon | 1 | 96,958 | 91,673 | 0.89% | 3,129 | 31 | City |
| St. Charles | 2 | 72,939 | 70,841 | 0.52% | 2,894 | 25 | City |
| St. Peters | 3 | 62,354 | 57,705 | 1.42% | 2,774 | 22 | City |
| Wentzville | 4 | 50,306 | 45,293 | 1.64% | 2,285 | 22 | City |
| Lake St. Louis | 5 | 19,707 | 16,804 | 3.02% | 2,308 | 9 | City |
| Dardenne Prairie | 6 | 15,425 | 12,780 | 2.61% | 2,892 | 5 | City |
| Cottleville | 7 | 6,881 | 5,698 | 2.44% | 1,454 | 5 | City |
| Weldon Spring | 8 | 5,290 | 5,335 | -0.21% | 692 | 8 | City |
| St. Paul | 9 | 4,011 | 3,060 | 4.21% | 557 | 7 | City |
| Foristell | 10 | 968 | 546 | 5.33% | 159 | 6 | City |
| New Melle | 11 | 549 | 552 | -0.18% | 307 | 2 | City |
| Josephville | 12 | 538 | 473 | 1.7% | 189 | 3 | Village |
| West Alton | 13 | 353 | 357 | -0.28% | 12 | 29 | City |
| Portage Des Sioux | 14 | 319 | 330 | -0.62% | 555 | 1 | City |
| Augusta | 15 | 264 | 269 | -0.38% | 256 | 1 | Town |
| Weldon Spring Heights | 16 | 90 | 94 | -1.1% | 938 | 0 | Town |
O'Fallon is the largest city in St. Charles County, with a population of 96,958.
Weldon Spring Heights is the smallest city in St. Charles County, with a population of 90.
There are 16 cities in St. Charles County, Missouri.