| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Le Mars | 1 | 10,760 | 10,603 | 0% | 1,225 | 9 | City |
| Remsen | 2 | 1,663 | 1,674 | 0% | 1,136 | 1 | City |
| Akron | 3 | 1,528 | 1,551 | -0.2% | 1,240 | 1 | City |
| Kingsley | 4 | 1,370 | 1,402 | -0.22% | 849 | 2 | City |
| Hinton | 5 | 953 | 933 | 0.53% | 1,065 | 1 | City |
| Merrill | 6 | 698 | 719 | -0.29% | 1,345 | 1 | City |
| Brunsville | 7 | 128 | 129 | 0% | 549 | 0 | City |
| Westfield | 8 | 118 | 122 | -0.84% | 894 | 0 | City |
| Oyens | 9 | 87 | 90 | -1.14% | 770 | 0 | City |
| Craig | 10 | 72 | 75 | -1.37% | 818 | 0 | City |
| Struble | 11 | 72 | 73 | -1.37% | 462 | 0 | City |
Le Mars is the largest city in Plymouth County, with a population of 10,760.
Struble is the smallest city in Plymouth County, with a population of 72.
There are 11 cities in Plymouth County, Iowa.