21 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Tiffin | 1 | 17,652 | 18,034 | -0.43% | 2,392 | 7 | City |
| Fostoria | 2 | 13,066 | 13,049 | 0.02% | 1,725 | 8 | City |
| Tiffin | 3 | 5,328 | 5,208 | 0.72% | 104 | 51 | Township |
| Loudon | 4 | 2,195 | 2,213 | 0.18% | 65 | 34 | Township |
| Liberty | 5 | 2,012 | 2,012 | 0.2% | 55 | 36 | Township |
| Eden | 6 | 2,001 | 2,008 | 0.2% | 55 | 36 | Township |
| Scipio | 7 | 1,710 | 1,691 | 0.88% | 46 | 37 | Township |
| Big Spring | 8 | 1,679 | 1,686 | 0.12% | 46 | 36 | Township |
| Venice | 9 | 1,649 | 1,663 | -0.06% | 42 | 40 | Township |
| Bloom | 10 | 1,627 | 1,649 | 0.06% | 45 | 36 | Township |
| Tiffin | 11 | 1,602 | 1,573 | 0.5% | 44 | 36 | Township |
| Pleasant | 12 | 1,489 | 1,497 | 0.13% | 42 | 36 | Township |
| Seneca | 13 | 1,437 | 1,457 | 0.07% | 40 | 36 | Township |
| Green Springs | 14 | 1,302 | 1,299 | 0% | 1,122 | 1 | Village |
| Adams | 15 | 1,219 | 1,212 | 0% | 34 | 36 | Township |
| Attica | 16 | 870 | 870 | 0% | 1,304 | 1 | Village |
| Bloomville | 17 | 850 | 867 | -0.47% | 1,410 | 1 | Village |
| Reed | 18 | 709 | 722 | -0.14% | 18 | 39 | Township |
| Bettsville | 19 | 585 | 588 | -0.17% | 1,123 | 1 | Village |
| Republic | 20 | 541 | 553 | -0.55% | 628 | 1 | Village |
| New Riegel | 21 | 278 | 285 | -0.71% | 1,390 | 0 | Village |
Tiffin is the largest city in Seneca County, with a population of 17,652.
New Riegel is the smallest city in Seneca County, with a population of 278.
There are 21 cities in Seneca County, Ohio.