8 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Tiffin | 1 | 17,576 | 18,034 | -0.43% | 2,382 | 7 | City |
| Fostoria | 2 | 13,070 | 13,049 | 0.03% | 1,726 | 8 | City |
| Green Springs | 3 | 1,303 | 1,299 | 0.08% | 1,123 | 1 | Village |
| Attica | 4 | 870 | 870 | 0% | 1,304 | 1 | Village |
| Bloomville | 5 | 847 | 867 | -0.35% | 1,405 | 1 | Village |
| Bettsville | 6 | 585 | 588 | 0% | 1,123 | 1 | Village |
| Republic | 7 | 539 | 553 | -0.37% | 625 | 1 | Village |
| New Riegel | 8 | 277 | 285 | -0.36% | 1,385 | 0 | Village |
Tiffin is the largest city in Seneca County, with a population of 17,576.
New Riegel is the smallest city in Seneca County, with a population of 277.
There are 8 cities in Seneca County, Ohio.