5 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marysville | 1 | 31,063 | 25,702 | 2.96% | 1,764 | 18 | City |
| Richwood | 2 | 2,745 | 2,234 | 3.2% | 2,210 | 1 | Village |
| Milford Center | 3 | 993 | 810 | 3.22% | 2,353 | 0 | Village |
| Magnetic Springs | 4 | 322 | 261 | 3.21% | 1,512 | 0 | Village |
| Unionville Center | 5 | 232 | 240 | -0.43% | 1,468 | 0 | Village |
Marysville is the largest city in Union County, with a population of 31,063.
Unionville Center is the smallest city in Union County, with a population of 232.
There are 5 cities in Union County, Ohio.