28 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 |
| Tiffin township | 3 | 5,345 | 5,205 | 0.89% | 104 | 51.18 | Township |
| Loudon township | 4 | 2,187 | 2,221 | -0.09% | 65 | 33.51 | Township |
| Liberty township | 5 | 2,019 | 2,020 | 0.3% | 56 | 36.34 | Township |
| Eden township | 6 | 2,000 | 2,006 | 0.25% | 55 | 36.32 | Township |
| Big Spring township | 7 | 1,675 | 1,684 | 0.18% | 46 | 36.42 | Township |
| Scipio township | 8 | 1,675 | 1,677 | 0.36% | 45 | 36.95 | Township |
| Venice township | 9 | 1,643 | 1,662 | -0.12% | 41 | 39.63 | Township |
| Bloom township | 10 | 1,617 | 1,649 | -0.19% | 44 | 36.40 | Township |
| Tiffin township | 11 | 1,596 | 1,573 | 0.57% | 44 | 36.35 | Township |
| Pleasant township | 12 | 1,482 | 1,497 | 0% | 42 | 35.53 | Township |
| Seneca township | 13 | 1,435 | 1,457 | 0% | 40 | 35.65 | Township |
| Adams township | 14 | 1,225 | 1,212 | 0.25% | 34 | 35.92 | Township |
| Green Springs | 15 | 1,218 | 1,228 | -0.16% | 1,050 | 1.16 | Village |
| Attica | 16 | 866 | 870 | -0.12% | 1,298 | 0.67 | Village |
| Bloomville | 17 | 843 | 868 | -0.59% | 1,398 | 0.60 | Village |
| Reed township | 18 | 707 | 723 | -0.28% | 18 | 38.61 | Township |
| Bettsville | 19 | 591 | 596 | -0.17% | 1,134 | 0.52 | Village |
| Republic | 20 | 534 | 553 | -0.74% | 619 | 0.86 | Village |
| McCutchenville | 21 | 348 | 211 | -0.85% | 125 | 2.78 | CDP |
| Flat Rock | 22 | 308 | 261 | 1.65% | 1,453 | 0.21 | CDP |
| New Riegel | 23 | 276 | 286 | -0.72% | 1,380 | 0.20 | Village |
| Bascom | 24 | 261 | 189 | -3.33% | 172 | 1.51 | CDP |
| Kansas | 25 | 185 | 52 | 0% | 421 | 0.44 | CDP |
| Old Fort | 26 | 126 | 218 | -3.08% | 218 | 0.58 | CDP |
| Fort Seneca | 27 | 84 | 102 | -11.58% | 134 | 0.63 | CDP |
| Melmore | 28 | 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 28 cities in Seneca County, Ohio.