| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marysville | 1 | 31,130 | 25,710 | 3.11% | 1,768 | 18 | City |
| Richwood | 2 | 2,727 | 2,231 | 3.3% | 2,196 | 1 | Village |
| Milford Center | 3 | 990 | 810 | 3.23% | 2,346 | 0 | Village |
| Magnetic Springs | 4 | 326 | 268 | 3.16% | 1,531 | 0 | Village |
| Unionville Center | 5 | 233 | 248 | -0.85% | 1,475 | 0 | Village |
Marysville is the largest city in Union County, with a population of 31,130.
Unionville Center is the smallest city in Union County, with a population of 233.
There are 5 cities in Union County, Ohio.