11 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Le Mars | 1 | 10,691 | 10,605 | 0.16% | 1,252 | 9 | City |
| Remsen | 2 | 1,683 | 1,671 | 0.12% | 1,150 | 1 | City |
| Akron | 3 | 1,544 | 1,549 | -0.06% | 1,253 | 1 | City |
| Kingsley | 4 | 1,392 | 1,402 | -0.14% | 862 | 2 | City |
| Hinton | 5 | 965 | 932 | 0.63% | 1,078 | 1 | City |
| Merrill | 6 | 711 | 719 | -0.28% | 1,370 | 1 | City |
| Brunsville | 7 | 128 | 127 | 0% | 549 | 0 | City |
| Westfield | 8 | 120 | 122 | -0.83% | 909 | 0 | City |
| Oyens | 9 | 87 | 89 | -1.14% | 770 | 0 | City |
| Craig | 10 | 78 | 77 | 0% | 886 | 0 | City |
| Struble | 11 | 71 | 73 | -1.39% | 455 | 0 | City |
Le Mars is the largest city in Plymouth County, with a population of 10,691.
Struble is the smallest city in Plymouth County, with a population of 71.
There are 11 cities in Plymouth County, Iowa.