6 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marysville | 1 | 30,234 | 25,706 | 3.09% | 1,857 | 16.28 | City |
| Richwood | 2 | 2,667 | 2,234 | 3.37% | 2,147 | 1.24 | Village |
| Milford Center | 3 | 965 | 809 | 3.32% | 2,287 | 0.42 | Village |
| Magnetic Springs | 4 | 314 | 261 | 3.63% | 1,474 | 0.21 | Village |
| Unionville Center | 5 | 230 | 239 | -0.86% | 1,456 | 0.16 | Village |
| Raymond | 6 | 130 | 155 | -5.8% | 102 | 1.27 | CDP |
Marysville is the largest city in Union County, with a population of 30,234.
Raymond is the smallest city in Union County.
There are 6 cities in Union County, Ohio.