10 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Lamoni | 1 | 1,985 | 1,957 | 0.3% | 502 | 3.95 | City |
| Leon | 2 | 1,821 | 1,811 | 0.11% | 581 | 3.13 | City |
| Grand River | 3 | 197 | 197 | 0% | 1,042 | 0.19 | City |
| Davis City | 4 | 180 | 177 | 0.56% | 310 | 0.58 | City |
| Van Wert | 5 | 180 | 177 | 0.56% | 506 | 0.36 | City |
| Decatur | 6 | 179 | 176 | 0.56% | 474 | 0.38 | City |
| Garden Grove | 7 | 175 | 172 | 0.57% | 259 | 0.68 | City |
| Weldon | 8 | 139 | 136 | 0.72% | 716 | 0.19 | City |
| Pleasanton | 9 | 30 | 30 | 0% | 85 | 0.35 | City |
| Le Roy | 10 | 12 | 12 | 0% | 32 | 0.38 | City |
Lamoni is the largest city in Decatur County, with a population of 1,985.
Le Roy is the smallest city in Decatur County.
There are 10 cities in Decatur County, Iowa.