| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Tiffin | 1 | 17,750 | 18,026 | -0.3% | 2,405 | 7 | City |
| Fostoria | 2 | 12,907 | 13,048 | -0.1% | 1,704 | 8 | City |
| Green Springs | 3 | 1,317 | 1,299 | 0.08% | 1,135 | 1 | Village |
| Attica | 4 | 862 | 870 | -0.12% | 1,292 | 1 | Village |
| Bloomville | 5 | 841 | 868 | -0.47% | 1,395 | 1 | Village |
| Bettsville | 6 | 583 | 591 | -0.17% | 1,119 | 1 | Village |
| Republic | 7 | 535 | 551 | -0.56% | 621 | 1 | Village |
| New Riegel | 8 | 277 | 286 | -0.72% | 1,385 | 0 | Village |
Tiffin is the largest city in Seneca County, with a population of 17,750.
New Riegel is the smallest city in Seneca County, with a population of 277.
There are 8 cities in Seneca County, Ohio.