19 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Harter | 1 | 5,877 | 6,130 | -0.93% | 105 | 56 | Township |
| Flora | 2 | 4,610 | 4,795 | -0.8% | 983 | 5 | City |
| Flora | 3 | 3,003 | 2,982 | 0.13% | 82 | 36 | Township |
| Louisville | 4 | 1,629 | 1,701 | -0.97% | 45 | 36 | Township |
| Louisville | 5 | 1,097 | 1,144 | -0.9% | 1,325 | 1 | Village |
| Clay City | 6 | 1,085 | 1,138 | -0.91% | 27 | 40 | Township |
| Clay City | 7 | 810 | 853 | -1.1% | 455 | 2 | Village |
| Xenia | 8 | 615 | 647 | -1.28% | 33 | 19 | Township |
| Blair | 9 | 590 | 621 | -1.01% | 16 | 36 | Township |
| Larkinsburg | 10 | 556 | 578 | -0.71% | 15 | 37 | Township |
| Pixley | 11 | 520 | 543 | -0.95% | 12 | 45 | Township |
| Xenia | 12 | 366 | 384 | -1.08% | 742 | 0 | Village |
| Songer | 13 | 332 | 349 | -1.19% | 9 | 37 | Township |
| Oskaloosa | 14 | 329 | 342 | -0.9% | 9 | 37 | Township |
| Hoosier | 15 | 304 | 319 | -0.98% | 9 | 36 | Township |
| Bible Grove | 16 | 286 | 303 | -1.04% | 8 | 35 | Township |
| La Clede | 17 | 215 | 33 | 6.97% | 700 | 0 | CDP |
| Iola | 18 | 95 | 100 | -1.04% | 99 | 1 | Village |
| Sailor Springs | 19 | 90 | 94 | -1.1% | 346 | 0 | Village |
Harter is the largest city in Clay County, with a population of 5,877.
Sailor Springs is the smallest city in Clay County, with a population of 90.
There are 19 cities in Clay County, Illinois.