15 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Monticello | 1 | 6,147 | 6,105 | -0.23% | 128 | 48 | Township |
| Monticello | 2 | 6,025 | 5,948 | 0.25% | 1,554 | 4 | City |
| Sangamon | 3 | 2,488 | 2,504 | -0.56% | 53 | 47 | Township |
| Cerro Gordo | 4 | 1,855 | 1,897 | -0.91% | 31 | 60 | Township |
| Bement | 5 | 1,597 | 1,641 | -0.87% | 33 | 48 | Township |
| Bement | 6 | 1,444 | 1,482 | -0.55% | 1,856 | 1 | Village |
| Blue Ridge | 7 | 1,358 | 1,385 | -0.73% | 21 | 64 | Township |
| Cerro Gordo | 8 | 1,289 | 1,317 | -0.46% | 1,768 | 1 | Village |
| Willow Branch | 9 | 945 | 959 | -0.74% | 14 | 67 | Township |
| Mansfield | 10 | 914 | 934 | -0.44% | 1,799 | 1 | Village |
| Goose Creek | 11 | 768 | 786 | -1.03% | 14 | 56 | Township |
| Hammond | 12 | 496 | 506 | -0.4% | 655 | 1 | Village |
| De Land | 13 | 440 | 450 | -0.45% | 1,038 | 0 | Village |
| Cisco | 14 | 250 | 258 | -0.79% | 1,374 | 0 | Village |
| White Heath | 15 | 141 | 153 | -6.62% | 413 | 0 | CDP |
Monticello is the largest city in Piatt County, with a population of 6,147.
White Heath is the smallest city in Piatt County, with a population of 141.
There are 15 cities in Piatt County, Illinois.