14 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Monticello | 1 | 6,133 | 6,105 | -0.23% | 128 | 48 | Township |
| Monticello | 2 | 6,041 | 5,948 | 0.27% | 1,558 | 4 | City |
| Sangamon | 3 | 2,474 | 2,504 | -0.56% | 53 | 47 | Township |
| Cerro Gordo | 4 | 1,838 | 1,897 | -0.92% | 30 | 60 | Township |
| Bement | 5 | 1,583 | 1,641 | -0.88% | 33 | 48 | Township |
| Bement | 6 | 1,437 | 1,482 | -0.48% | 1,847 | 1 | Village |
| Blue Ridge | 7 | 1,348 | 1,385 | -0.74% | 21 | 64 | Township |
| Cerro Gordo | 8 | 1,284 | 1,317 | -0.39% | 1,761 | 1 | Village |
| Willow Branch | 9 | 938 | 959 | -0.74% | 14 | 67 | Township |
| Mansfield | 10 | 910 | 934 | -0.44% | 1,791 | 1 | Village |
| Goose Creek | 11 | 760 | 786 | -1.04% | 13 | 56 | Township |
| Hammond | 12 | 494 | 506 | -0.4% | 653 | 1 | Village |
| De Land | 13 | 438 | 450 | -0.45% | 1,033 | 0 | Village |
| Cisco | 14 | 249 | 258 | -0.4% | 1,368 | 0 | Village |
Monticello is the largest city in Piatt County, with a population of 6,133.
Cisco is the smallest city in Piatt County, with a population of 249.
There are 14 cities in Piatt County, Illinois.