15 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Tiffin | 1 | 17,360 | 17,967 | -0.69% | 2,400 | 7.23 | City |
| Fostoria | 2 | 13,036 | 13,043 | -0.01% | 1,721 | 7.57 | City |
| Green Springs | 3 | 1,218 | 1,228 | -0.16% | 1,050 | 1.16 | Village |
| Attica | 4 | 866 | 870 | -0.11% | 1,298 | 0.67 | Village |
| Bloomville | 5 | 843 | 868 | -0.59% | 1,398 | 0.60 | Village |
| Bettsville | 6 | 591 | 596 | -0.17% | 1,134 | 0.52 | Village |
| Republic | 7 | 534 | 553 | -0.74% | 619 | 0.86 | Village |
| McCutchenville | 8 | 348 | 211 | -0.85% | 125 | 2.78 | CDP |
| Flat Rock | 9 | 308 | 261 | 1.65% | 1,453 | 0.21 | CDP |
| New Riegel | 10 | 276 | 286 | -0.72% | 1,380 | 0.20 | Village |
| Bascom | 11 | 261 | 189 | -3.33% | 172 | 1.51 | CDP |
| Kansas | 12 | 185 | 52 | 0% | 421 | 0.44 | CDP |
| Old Fort | 13 | 126 | 218 | -3.08% | 218 | 0.58 | CDP |
| Fort Seneca | 14 | 84 | 102 | -11.58% | 134 | 0.63 | CDP |
| Melmore | 15 | 66 | 62 | -7.04% | 111 | 0.59 | CDP |
Tiffin is the largest city in Seneca County, with a population of 17,360.
Melmore is the smallest city in Seneca County.
There are 15 cities in Seneca County, Ohio.