15 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Monticello township | 1 | 6,239 | 6,103 | 0.56% | 130 | 48 | Township |
| Monticello | 2 | 6,063 | 5,950 | 0.38% | 1,563 | 3.88 | City |
| Sangamon township | 3 | 2,536 | 2,509 | 0.32% | 54 | 47.12 | Township |
| Cerro Gordo township | 4 | 1,889 | 1,897 | 0% | 31 | 60.30 | Township |
| Bement township | 5 | 1,619 | 1,640 | -0.18% | 34 | 48.31 | Township |
| Bement | 6 | 1,457 | 1,484 | -0.34% | 1,873 | 0.78 | Village |
| Blue Ridge township | 7 | 1,376 | 1,386 | -0.07% | 22 | 63.64 | Township |
| Cerro Gordo | 8 | 1,300 | 1,315 | -0.23% | 1,783 | 0.73 | Village |
| Willow Branch township | 9 | 956 | 959 | -0.1% | 14 | 67.33 | Township |
| Mansfield | 10 | 922 | 936 | -0.32% | 1,815 | 0.51 | Village |
| Goose Creek township | 11 | 785 | 785 | 0.13% | 14 | 56.35 | Township |
| Hammond | 12 | 497 | 506 | -0.4% | 657 | 0.76 | Village |
| De Land | 13 | 441 | 450 | -0.45% | 1,040 | 0.42 | Village |
| Cisco | 14 | 251 | 255 | -0.4% | 1,379 | 0.18 | Village |
| White Heath | 15 | 141 | 153 | -6.62% | 413 | 0.34 | CDP |
Monticello township is the largest city in Piatt County, with a population of 6,239.
White Heath is the smallest city in Piatt County.
There are 15 cities in Piatt County, Illinois.