35 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| DeKalb | 1 | 43,120 | 42,643 | 0.44% | 1,229 | 35 | Township |
| DeKalb | 2 | 40,557 | 40,224 | 0.16% | 2,340 | 17 | City |
| Sycamore | 3 | 19,041 | 18,675 | 0.38% | 1,881 | 10 | City |
| Sycamore | 4 | 15,016 | 14,789 | 0.56% | 438 | 34 | Township |
| Pingree Grove | 5 | 11,790 | 10,398 | 2.41% | 2,853 | 4 | Village |
| Cortland | 6 | 11,612 | 11,183 | 1.12% | 331 | 35 | Township |
| Sandwich | 7 | 7,664 | 7,515 | 0.92% | 502 | 15 | Township |
| Sandwich | 8 | 7,321 | 7,211 | 0.3% | 1,546 | 5 | City |
| Genoa | 9 | 5,759 | 5,539 | 0.93% | 160 | 36 | Township |
| Lake Holiday | 10 | 5,666 | 5,365 | 1.07% | 1,617 | 4 | CDP |
| Genoa | 11 | 5,565 | 5,307 | 0.92% | 2,144 | 3 | City |
| Cortland | 12 | 4,641 | 4,396 | 1.07% | 1,292 | 4 | Town |
| Kingston | 13 | 3,502 | 3,468 | 0.52% | 98 | 36 | Township |
| Squaw Grove | 14 | 2,702 | 2,725 | 0.15% | 77 | 35 | Township |
| Somonauk | 15 | 1,973 | 1,997 | -0.05% | 102 | 19 | Township |
| Hinckley | 16 | 1,971 | 1,996 | -0.25% | 2,300 | 1 | Village |
| Somonauk | 17 | 1,755 | 1,782 | -0.34% | 702 | 3 | Village |
| Kirkland | 18 | 1,614 | 1,646 | -0.43% | 1,300 | 1 | Village |
| Waterman | 19 | 1,571 | 1,430 | 1.81% | 1,049 | 1 | Village |
| Malta | 20 | 1,526 | 1,533 | 0.13% | 44 | 35 | Township |
| Shabbona | 21 | 1,335 | 1,351 | 0.15% | 39 | 35 | Township |
| Hillcrest | 22 | 1,202 | 1,222 | -0.33% | 339 | 4 | Village |
| Malta | 23 | 1,135 | 1,143 | -0.18% | 1,846 | 1 | Village |
| Kingston | 24 | 1,092 | 1,105 | -0.27% | 1,110 | 1 | Village |
| Lily Lake | 25 | 1,064 | 1,037 | 0.47% | 392 | 3 | Village |
| Afton | 26 | 907 | 905 | 0.33% | 26 | 35 | Township |
| Mayfield | 27 | 907 | 910 | 0.33% | 26 | 35 | Township |
| Shabbona | 28 | 848 | 866 | -0.47% | 661 | 1 | Village |
| Kings | 29 | 434 | 348 | 7.16% | 2,837 | 0 | CDP |
| Pierce | 30 | 430 | 438 | -0.46% | 12 | 35 | Township |
| South Grove | 31 | 415 | 421 | 0% | 12 | 34 | Township |
| Victor | 32 | 288 | 290 | 0.35% | 9 | 33 | Township |
| Virgil | 33 | 283 | 288 | -0.35% | 130 | 2 | Village |
| Kingston | 34 | 148 | 107 | 7.25% | 695 | 0 | CDP |
| Harding | 35 | 138 | 153 | 7.81% | 812 | 0 | CDP |
DeKalb is the largest city in DeKalb County, with a population of 43,120.
Harding is the smallest city in DeKalb County, with a population of 138.
There are 35 cities in DeKalb County, Illinois.