33 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| O'Fallon | 1 | 96,277 | 91,375 | 0.97% | 3,154 | 31 | City |
| O'Fallon | 2 | 96,277 | 91,665 | 0.97% | 3,154 | 31 | City |
| St. Charles | 3 | 72,875 | 70,586 | 0.58% | 2,892 | 25 | City |
| St. Charles | 4 | 72,875 | 70,790 | 0.58% | 2,892 | 25 | City |
| St. Peters | 5 | 62,320 | 57,729 | 1.5% | 2,772 | 22 | City |
| St. Peters | 6 | 62,320 | 57,707 | 1.5% | 2,772 | 22 | City |
| Wentzville | 7 | 49,487 | 44,932 | 1.73% | 2,317 | 21 | City |
| Wentzville | 8 | 49,487 | 45,282 | 1.73% | 2,317 | 21 | City |
| Lake St. Louis | 9 | 19,752 | 16,707 | 3.09% | 2,313 | 9 | City |
| Lake St. Louis | 10 | 19,752 | 16,789 | 3.09% | 2,313 | 9 | City |
| Dardenne Prairie | 11 | 14,813 | 12,764 | 2.82% | 2,777 | 5 | City |
| Dardenne Prairie | 12 | 14,813 | 12,780 | 2.82% | 2,777 | 5 | City |
| Cottleville | 13 | 6,547 | 5,621 | 2.65% | 1,383 | 5 | City |
| Cottleville | 14 | 6,547 | 5,701 | 2.65% | 1,383 | 5 | City |
| Weldon Spring | 15 | 5,296 | 5,328 | -0.15% | 693 | 8 | City |
| Weldon Spring | 16 | 5,296 | 5,333 | -0.15% | 693 | 8 | City |
| St. Paul | 17 | 3,891 | 3,005 | 4.46% | 540 | 7 | City |
| St. Paul | 18 | 3,891 | 3,061 | 4.46% | 540 | 7 | City |
| Foristell | 19 | 801 | 546 | 6.66% | 132 | 6 | City |
| Foristell | 20 | 801 | 550 | 6.66% | 132 | 6 | City |
| New Melle | 21 | 546 | 550 | -0.18% | 324 | 2 | City |
| New Melle | 22 | 546 | 549 | -0.18% | 324 | 2 | City |
| Josephville | 23 | 526 | 477 | 1.74% | 185 | 3 | Village |
| Josephville | 24 | 526 | 479 | 1.74% | 185 | 3 | Village |
| West Alton | 25 | 352 | 356 | -0.28% | 12 | 29 | City |
| West Alton | 26 | 352 | 356 | -0.28% | 12 | 29 | City |
| Portage Des Sioux | 27 | 321 | 331 | -0.62% | 558 | 1 | City |
| Portage Des Sioux | 28 | 321 | 331 | -0.62% | 558 | 1 | City |
| Augusta | 29 | 267 | 271 | -0.37% | 303 | 1 | Town |
| Augusta | 30 | 267 | 271 | -0.37% | 303 | 1 | Town |
| Weldon Spring Heights | 31 | 91 | 94 | -1.09% | 948 | 0 | Town |
| Weldon Spring Heights | 32 | 91 | 94 | -1.09% | 948 | 0 | Town |
| Defiance | 33 | 62 | 68 | -10.14% | 61 | 1 | CDP |
O'Fallon is the largest city in St. Charles County, with a population of 96,277.
Defiance is the smallest city in St. Charles County, with a population of 62.
There are 33 cities in St. Charles County, Missouri.