22 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Union township | 1 | 14,330 | 13,708 | 0.94% | 805 | 17.80 | Township |
| Auburn | 2 | 14,127 | 13,529 | 0.86% | 1,747 | 8.09 | City |
| Keyser township | 3 | 8,126 | 7,859 | 0.79% | 339 | 23.95 | Township |
| Garrett | 4 | 6,739 | 6,544 | 0.58% | 1,618 | 4.17 | City |
| Wilmington township | 5 | 4,132 | 3,995 | 0.83% | 115 | 35.92 | Township |
| Jackson township | 6 | 3,835 | 3,701 | 0.81% | 108 | 35.38 | Township |
| Grant township | 7 | 3,190 | 3,078 | 0.85% | 183 | 17.44 | Township |
| Butler | 8 | 2,707 | 2,636 | 0.52% | 1,333 | 2.03 | City |
| Waterloo | 9 | 2,187 | 2,112 | 0.69% | 1,036 | 2.11 | Town |
| Butler township | 10 | 1,779 | 1,718 | 0.79% | 75 | 23.69 | Township |
| Smithfield township | 11 | 1,772 | 1,715 | 0.8% | 51 | 34.99 | Township |
| Fairfield township | 12 | 1,423 | 1,363 | 0.99% | 40 | 35.66 | Township |
| Concord township | 13 | 1,418 | 1,372 | 0.78% | 79 | 17.99 | Township |
| Butler township | 14 | 1,315 | 1,309 | -0.15% | 43 | 30.28 | Township |
| Ashley | 15 | 1,053 | 1,028 | 0.48% | 553 | 1.90 | Town |
| Butler township | 16 | 756 | 798 | -1.56% | 26 | 28.91 | Township |
| Newville township | 17 | 596 | 573 | 1.02% | 43 | 13.94 | Township |
| Waterloo township | 18 | 526 | 527 | -0.19% | 31 | 17.15 | Township |
| St. Joe | 19 | 429 | 414 | 0.7% | 1,281 | 0.34 | Town |
| Stafford township | 20 | 285 | 275 | 0.71% | 20 | 14.61 | Township |
| Corunna | 21 | 242 | 236 | 0.41% | 1,352 | 0.18 | Town |
| Altona | 22 | 220 | 214 | 0.46% | 1,084 | 0.20 | Town |
Union township is the largest city in DeKalb County, with a population of 14,330.
Altona is the smallest city in DeKalb County.
There are 22 cities in DeKalb County, Indiana.