| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Waterloo | 1 | 11,182 | 10,994 | 0.45% | 1,358 | 8 | City |
| Columbia | 2 | 10,845 | 11,039 | -0.18% | 1,028 | 11 | City |
| Valmeyer | 3 | 1,201 | 1,225 | -0.25% | 351 | 3 | Village |
| Prairie du Rocher | 4 | 513 | 499 | 0.59% | 898 | 1 | Village |
| Hecker | 5 | 416 | 430 | -0.48% | 1,558 | 0 | Village |
| Ruma | 6 | 312 | 317 | -0.32% | 420 | 1 | Village |
| Maeystown | 7 | 150 | 152 | -0.66% | 508 | 0 | Village |
| Fults | 8 | 26 | 28 | -3.7% | 491 | 0 | Village |
Waterloo is the largest city in Monroe County, with a population of 11,182.
Fults is the smallest city in Monroe County, with a population of 26.
There are 8 cities in Monroe County, Illinois.