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 |
|---|---|---|---|---|---|---|---|
| O'Fallon | 1 | 94,891 | 91,621 | 0.87% | 3,109 | 30.5 | City |
| St. Charles | 2 | 72,140 | 70,780 | 0.47% | 2,863 | 25.2 | City |
| St. Peters | 3 | 59,981 | 57,708 | 0.96% | 2,668 | 22.5 | City |
| Wentzville | 4 | 48,342 | 44,962 | 1.78% | 2,263 | 21.4 | City |
| Lake St. Louis | 5 | 19,285 | 16,787 | 3.34% | 2,258 | 8.5 | City |
| Dardenne Prairie | 6 | 14,146 | 12,774 | 2.48% | 2,652 | 5.3 | City |
| Cottleville | 7 | 6,158 | 5,696 | 1.9% | 1,301 | 4.7 | City |
| Weldon Spring | 8 | 5,240 | 5,332 | -0.44% | 685 | 7.6 | City |
| St. Paul | 9 | 3,809 | 3,060 | 5.16% | 529 | 7.2 | City |
| Foristell | 10 | 637 | 551 | 3.41% | 105 | 6.1 | City |
| Josephville | 11 | 552 | 516 | 1.66% | 194 | 2.9 | Village |
| New Melle | 12 | 536 | 544 | -0.37% | 318 | 1.7 | City |
| West Alton | 13 | 348 | 356 | -0.57% | 12 | 28.8 | City |
| Portage Des Sioux | 14 | 322 | 333 | -0.92% | 560 | 0.6 | City |
| Augusta | 15 | 262 | 269 | -0.76% | 298 | 0.9 | Town |
| Weldon Spring Heights | 16 | 91 | 94 | -1.09% | 948 | 0.1 | Town |
| Defiance | 17 | 69 | 68 | -8% | 68 | 1.0 | CDP |
| Three Creeks | 18 | 6 | 6 | 0% | 1 | 4.8 | 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.
O'Fallon is the largest city in St. Charles County.
Three Creeks is the smallest city in St. Charles County.
There are 18 cities in St. Charles County, Missouri.