9 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Waterloo | 1 | 11,236 | 11,005 | 0.41% | 1,365 | 8 | City |
| Columbia | 2 | 10,866 | 11,033 | -0.3% | 1,030 | 11 | City |
| Valmeyer | 3 | 1,201 | 1,228 | -0.41% | 351 | 3 | Village |
| Prairie Rocher | 4 | 515 | 500 | 0.59% | 902 | 1 | Village |
| Hecker | 5 | 411 | 428 | -0.72% | 1,539 | 0 | Village |
| Ruma | 6 | 308 | 312 | -0.32% | 415 | 1 | Village |
| Maeystown | 7 | 147 | 149 | 0% | 498 | 0 | Village |
| Floraville | 8 | 27 | 30 | 3.85% | 342 | 0 | CDP |
| Fults | 9 | 24 | 26 | 0% | 453 | 0 | Village |
Waterloo is the largest city in Monroe County, with a population of 11,236.
Fults is the smallest city in Monroe County, with a population of 24.
There are 9 cities in Monroe County, Illinois.