8 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Waterloo | 1 | 11,245 | 11,004 | 0.36% | 1,366 | 8 | City |
| Columbia | 2 | 10,861 | 11,034 | -0.27% | 1,029 | 11 | City |
| Valmeyer | 3 | 1,207 | 1,228 | -0.25% | 353 | 3 | Village |
| Prairie du Rocher | 4 | 515 | 499 | 0.59% | 902 | 1 | Village |
| Hecker | 5 | 416 | 430 | -0.48% | 1,558 | 0 | Village |
| Ruma | 6 | 308 | 313 | -0.32% | 415 | 1 | Village |
| Maeystown | 7 | 148 | 151 | 0% | 502 | 0 | Village |
| Fults | 8 | 25 | 27 | 0% | 472 | 0 | Village |
Waterloo is the largest city in Monroe County, with a population of 11,245.
Fults is the smallest city in Monroe County, with a population of 25.
There are 8 cities in Monroe County, Illinois.