16 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marysville | 1 | 30,234 | 25,706 | 3.09% | 1,857 | 16 | City |
| Jerome | 2 | 11,449 | 9,572 | 3.62% | 338 | 34 | Township |
| Claibourne | 3 | 4,216 | 3,504 | 3.74% | 122 | 34 | Township |
| Allen | 4 | 2,915 | 2,409 | 3.88% | 98 | 30 | Township |
| Darby | 5 | 2,865 | 2,424 | 3.47% | 92 | 31 | Township |
| Richwood | 6 | 2,667 | 2,234 | 3.37% | 2,147 | 1 | Village |
| Liberty | 7 | 2,517 | 2,081 | 3.88% | 69 | 36 | Township |
| Union | 8 | 2,051 | 1,701 | 3.8% | 58 | 36 | Township |
| Taylor | 9 | 1,875 | 1,553 | 3.82% | 71 | 26 | Township |
| York | 10 | 1,622 | 1,334 | 3.97% | 44 | 37 | Township |
| Millcreek | 11 | 1,598 | 1,324 | 3.83% | 77 | 21 | Township |
| Washington | 12 | 971 | 806 | 3.74% | 35 | 28 | Township |
| Milford Center | 13 | 965 | 809 | 3.32% | 2,287 | 0 | Village |
| Magnetic Springs | 14 | 314 | 261 | 3.63% | 1,474 | 0 | Village |
| Unionville Center | 15 | 230 | 239 | -0.86% | 1,456 | 0 | Village |
| Raymond | 16 | 130 | 155 | -5.8% | 102 | 1 | CDP |
Marysville is the largest city in Union County, with a population of 30,234.
Raymond is the smallest city in Union County, with a population of 130.
There are 16 cities in Union County, Ohio.