15 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 |
| Jerome | 2 | 11,622 | 9,574 | 2.8% | 343 | 34 | Township |
| Claibourne | 3 | 4,290 | 3,505 | 2.95% | 125 | 34 | Township |
| Allen | 4 | 2,965 | 2,408 | 3.02% | 100 | 30 | Township |
| Darby | 5 | 2,904 | 2,425 | 2.61% | 93 | 31 | Township |
| Richwood | 6 | 2,745 | 2,234 | 3.2% | 2,210 | 1 | Village |
| Liberty | 7 | 2,561 | 2,081 | 3.06% | 71 | 36 | Township |
| Union | 8 | 2,074 | 1,701 | 2.83% | 58 | 36 | Township |
| Taylor | 9 | 1,913 | 1,556 | 3.02% | 73 | 26 | Township |
| York | 10 | 1,642 | 1,334 | 3.08% | 44 | 37 | Township |
| Millcreek | 11 | 1,628 | 1,324 | 3.04% | 78 | 21 | Township |
| Milford Center | 12 | 993 | 810 | 3.22% | 2,353 | 0 | Village |
| Washington | 13 | 990 | 806 | 3.02% | 36 | 28 | Township |
| Magnetic Springs | 14 | 322 | 261 | 3.21% | 1,512 | 0 | Village |
| Unionville Center | 15 | 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 15 cities in Union County, Ohio.