22 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Union | 1 | 14,265 | 13,711 | 0.78% | 802 | 18 | Township |
| Auburn | 2 | 14,102 | 13,537 | 0.81% | 1,744 | 8 | City |
| Keyser | 3 | 8,017 | 7,867 | 0.12% | 335 | 24 | Township |
| Garrett | 4 | 6,691 | 6,548 | 0.42% | 1,606 | 4 | City |
| Wilmington | 5 | 4,062 | 3,995 | 0.05% | 113 | 36 | Township |
| Jackson | 6 | 3,792 | 3,705 | 0.16% | 107 | 35 | Township |
| Grant | 7 | 3,142 | 3,079 | 0.13% | 180 | 17 | Township |
| Butler | 8 | 2,679 | 2,628 | 0.37% | 1,320 | 2 | City |
| Waterloo | 9 | 2,168 | 2,112 | 0.51% | 1,027 | 2 | Town |
| Butler | 10 | 1,759 | 1,718 | 0.23% | 74 | 24 | Township |
| Smithfield | 11 | 1,753 | 1,715 | 0.23% | 50 | 35 | Township |
| Fairfield | 12 | 1,404 | 1,358 | 0.43% | 39 | 36 | Township |
| Concord | 13 | 1,396 | 1,368 | 0.07% | 78 | 18 | Township |
| Butler | 14 | 1,324 | 1,312 | -0.15% | 44 | 30 | Township |
| Ashley | 15 | 1,044 | 1,027 | 0.29% | 548 | 2 | Town |
| Butler | 16 | 784 | 797 | 0.38% | 27 | 29 | Township |
| Newville | 17 | 580 | 573 | -0.34% | 42 | 14 | Township |
| Waterloo | 18 | 524 | 525 | 0.19% | 31 | 17 | Township |
| St. Joe | 19 | 418 | 410 | 0.24% | 1,248 | 0 | Town |
| Stafford | 20 | 282 | 278 | -0.35% | 19 | 15 | Township |
| Corunna | 21 | 239 | 236 | 0% | 1,335 | 0 | Town |
| Altona | 22 | 220 | 215 | 0.46% | 1,084 | 0 | Town |
Union is the largest city in DeKalb County, with a population of 14,265.
Altona is the smallest city in DeKalb County, with a population of 220.
There are 22 cities in DeKalb County, Indiana.