| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Lamoni | 1 | 2,072 | 2,079 | -0.1% | 524 | 4 | City |
| Leon | 2 | 1,768 | 1,805 | -0.28% | 564 | 3 | City |
| Grand River | 3 | 188 | 193 | -0.53% | 995 | 0 | City |
| Decatur | 4 | 178 | 174 | 0% | 471 | 0 | City |
| Davis City | 5 | 177 | 180 | -0.56% | 305 | 1 | City |
| Van Wert | 6 | 174 | 177 | -0.57% | 489 | 0 | City |
| Garden Grove | 7 | 173 | 176 | -0.57% | 256 | 1 | City |
| Weldon | 8 | 135 | 140 | -0.74% | 696 | 0 | City |
| Pleasanton | 9 | 32 | 33 | -3.03% | 91 | 0 | City |
| Le Roy | 10 | 10 | 10 | 0% | 26 | 0 | City |
Lamoni is the largest city in Decatur County, with a population of 2,072.
Le Roy is the smallest city in Decatur County, with a population of 10.
There are 10 cities in Decatur County, Iowa.