20 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| DeKalb | 1 | 40,205 | 40,219 | -0.01% | 2,320 | 17.33 | City |
| Sycamore | 2 | 18,752 | 18,576 | 0.19% | 1,853 | 10.12 | City |
| Pingree Grove | 3 | 11,629 | 10,399 | 2.16% | 2,814 | 4.13 | Village |
| Sandwich | 4 | 7,212 | 7,212 | 0% | 1,523 | 4.74 | City |
| Lake Holiday | 5 | 5,666 | 5,365 | 1.07% | 1,617 | 3.50 | CDP |
| Genoa | 6 | 5,523 | 5,307 | 0.78% | 2,128 | 2.60 | City |
| Cortland | 7 | 4,548 | 4,397 | 0.66% | 1,266 | 3.59 | Town |
| Hinckley | 8 | 1,953 | 1,997 | -0.46% | 2,279 | 0.86 | Village |
| Somonauk | 9 | 1,743 | 1,785 | -0.46% | 697 | 2.50 | Village |
| Kirkland | 10 | 1,598 | 1,648 | -0.62% | 1,287 | 1.24 | Village |
| Waterman | 11 | 1,413 | 1,433 | -0.28% | 943 | 1.50 | Village |
| Hillcrest | 12 | 1,207 | 1,221 | -0.25% | 340 | 3.55 | Village |
| Malta | 13 | 1,125 | 1,142 | -0.27% | 1,829 | 0.61 | Village |
| Kingston | 14 | 1,077 | 1,106 | -0.55% | 1,095 | 0.98 | Village |
| Lily Lake | 15 | 1,045 | 1,035 | 0.19% | 385 | 2.72 | Village |
| Shabbona | 16 | 840 | 865 | -0.59% | 655 | 1.28 | Village |
| Kings | 17 | 434 | 348 | 7.16% | 2,837 | 0.15 | CDP |
| Virgil | 18 | 279 | 289 | -0.71% | 128 | 2.18 | Village |
| Kingston | 19 | 148 | 107 | 7.25% | 695 | 0.21 | CDP |
| Harding | 20 | 138 | 153 | 7.81% | 812 | 0.17 | CDP |
DeKalb is the largest city in DeKalb County, with a population of 40,205.
Harding is the smallest city in DeKalb County.
There are 20 cities in DeKalb County, Illinois.