| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| DeKalb | 1 | 40,424 | 40,242 | -0.11% | 2,268 | 18 | City |
| Sycamore | 2 | 19,135 | 18,649 | 0.27% | 1,890 | 10 | City |
| Pingree Grove | 3 | 12,942 | 10,419 | 3.16% | 3,203 | 4 | Village |
| Sandwich | 4 | 7,394 | 7,212 | 0.19% | 1,561 | 5 | City |
| Genoa | 5 | 5,591 | 5,304 | 0.79% | 2,154 | 3 | City |
| Cortland | 6 | 4,718 | 4,396 | 0.9% | 1,313 | 4 | Town |
| Hinckley | 7 | 1,989 | 1,998 | -0.35% | 2,321 | 1 | Village |
| Somonauk | 8 | 1,762 | 1,791 | -0.45% | 705 | 3 | Village |
| Waterman | 9 | 1,666 | 1,431 | 1.59% | 1,112 | 1 | Village |
| Kirkland | 10 | 1,624 | 1,651 | -0.49% | 1,308 | 1 | Village |
| Hillcrest | 11 | 1,205 | 1,227 | -0.41% | 340 | 4 | Village |
| Malta | 12 | 1,137 | 1,143 | -0.26% | 1,849 | 1 | Village |
| Kingston | 13 | 1,099 | 1,108 | -0.36% | 1,117 | 1 | Village |
| Lily Lake | 14 | 1,080 | 1,033 | 0.65% | 397 | 3 | Village |
| Shabbona | 15 | 863 | 879 | -0.46% | 673 | 1 | Village |
| Virgil | 16 | 283 | 289 | -0.35% | 130 | 2 | Village |
DeKalb is the largest city in DeKalb County, with a population of 40,424.
Virgil is the smallest city in DeKalb County, with a population of 283.
There are 16 cities in DeKalb County, Illinois.