16 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marysville | 1 | 30,169 | 25,702 | 3.05% | 1,853 | 16 | City |
| Jerome | 2 | 11,305 | 9,574 | 2.88% | 333 | 34 | Township |
| Claibourne | 3 | 4,167 | 3,505 | 3.04% | 121 | 34 | Township |
| Allen | 4 | 2,878 | 2,408 | 3.12% | 97 | 30 | Township |
| Darby | 5 | 2,830 | 2,425 | 2.69% | 91 | 31 | Township |
| Richwood | 6 | 2,660 | 2,234 | 3.3% | 2,142 | 1 | Village |
| Liberty | 7 | 2,485 | 2,081 | 3.15% | 69 | 36 | Township |
| Union | 8 | 2,017 | 1,701 | 2.91% | 57 | 36 | Township |
| Taylor | 9 | 1,857 | 1,556 | 3.11% | 71 | 26 | Township |
| York | 10 | 1,593 | 1,334 | 3.17% | 43 | 37 | Township |
| Millcreek | 11 | 1,580 | 1,324 | 3.13% | 76 | 21 | Township |
| Milford Center | 12 | 962 | 810 | 3.22% | 2,280 | 0 | Village |
| Washington | 13 | 961 | 806 | 3.11% | 35 | 28 | Township |
| Magnetic Springs | 14 | 312 | 261 | 3.31% | 1,465 | 0 | Village |
| Unionville Center | 15 | 233 | 240 | -0.85% | 1,475 | 0 | Village |
| Raymond | 16 | 130 | 155 | -5.8% | 102 | 1 | CDP |
Marysville is the largest city in Union County, with a population of 30,169.
Raymond is the smallest city in Union County, with a population of 130.
There are 16 cities in Union County, Ohio.