11 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Le Mars | 1 | 10,629 | 10,601 | 0.06% | 1,245 | 8.54 | City |
| Remsen | 2 | 1,687 | 1,674 | 0.18% | 1,152 | 1.46 | City |
| Akron | 3 | 1,536 | 1,550 | -0.19% | 1,247 | 1.23 | City |
| Kingsley | 4 | 1,384 | 1,401 | -0.22% | 857 | 1.61 | City |
| Hinton | 5 | 969 | 934 | 0.73% | 1,083 | 0.90 | City |
| Merrill | 6 | 705 | 717 | -0.28% | 1,358 | 0.52 | City |
| Brunsville | 7 | 129 | 128 | 0% | 554 | 0.23 | City |
| Westfield | 8 | 119 | 121 | 0% | 902 | 0.13 | City |
| Oyens | 9 | 93 | 95 | 0% | 823 | 0.11 | City |
| Craig | 10 | 75 | 77 | 0% | 852 | 0.09 | City |
| Struble | 11 | 66 | 68 | 0% | 423 | 0.16 | City |
Le Mars is the largest city in Plymouth County, with a population of 10,629.
Struble is the smallest city in Plymouth County, with a population of 66.
There are 11 cities in Plymouth County, Iowa.