18 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Harter | 1 | 5,822 | 6,130 | -0.94% | 104 | 56 | Township |
| Flora | 2 | 4,573 | 4,795 | -0.8% | 975 | 5 | City |
| Flora | 3 | 3,007 | 2,982 | 0.13% | 82 | 36 | Township |
| Louisville | 4 | 1,613 | 1,701 | -0.98% | 44 | 36 | Township |
| Louisville | 5 | 1,088 | 1,144 | -0.82% | 1,314 | 1 | Village |
| Clay City | 6 | 1,075 | 1,138 | -0.92% | 27 | 40 | Township |
| Clay City | 7 | 802 | 853 | -0.99% | 450 | 2 | Village |
| Xenia | 8 | 607 | 647 | -1.3% | 33 | 19 | Township |
| Blair | 9 | 584 | 621 | -1.02% | 16 | 36 | Township |
| Larkinsburg | 10 | 552 | 578 | -0.72% | 15 | 37 | Township |
| Pixley | 11 | 515 | 543 | -0.96% | 11 | 45 | Township |
| Xenia | 12 | 363 | 384 | -0.82% | 736 | 0 | Village |
| Songer | 13 | 328 | 349 | -1.2% | 9 | 37 | Township |
| Oskaloosa | 14 | 326 | 342 | -0.91% | 9 | 37 | Township |
| Hoosier | 15 | 301 | 319 | -0.99% | 8 | 36 | Township |
| Bible Grove | 16 | 283 | 303 | -1.05% | 8 | 35 | Township |
| Iola | 17 | 94 | 100 | -1.05% | 98 | 1 | Village |
| Sailor Springs | 18 | 89 | 94 | -1.11% | 342 | 0 | Village |
Harter is the largest city in Clay County, with a population of 5,822.
Sailor Springs is the smallest city in Clay County, with a population of 89.
There are 18 cities in Clay County, Illinois.