9 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Waterloo | 1 | 11,205 | 11,004 | 0.36% | 1,361 | 8 | City |
| Columbia | 2 | 10,890 | 11,034 | -0.27% | 1,032 | 11 | City |
| Valmeyer | 3 | 1,210 | 1,228 | -0.33% | 353 | 3 | Village |
| Prairie Rocher | 4 | 512 | 499 | 0.39% | 897 | 1 | Village |
| Hecker | 5 | 418 | 430 | -0.71% | 1,566 | 0 | Village |
| Ruma | 6 | 309 | 313 | -0.32% | 416 | 1 | Village |
| Maeystown | 7 | 148 | 151 | -0.67% | 502 | 0 | Village |
| Floraville | 8 | 27 | 30 | 3.85% | 342 | 0 | CDP |
| Fults | 9 | 25 | 27 | -3.85% | 472 | 0 | Village |
Waterloo is the largest city in Monroe County, with a population of 11,205.
Fults is the smallest city in Monroe County, with a population of 25.
There are 9 cities in Monroe County, Illinois.