26 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Liberty | 1 | 11,128 | 10,919 | 0.16% | 449 | 25 | Township |
| Liberty | 2 | 6,329 | 6,065 | 0.72% | 128 | 49 | Township |
| Liberty | 3 | 4,776 | 4,774 | 0.06% | 151 | 32 | Township |
| Liberty | 4 | 4,499 | 4,424 | 0.47% | 129 | 35 | Township |
| Liberty | 5 | 3,420 | 3,403 | 0% | 80 | 43 | Township |
| Center | 6 | 2,786 | 2,913 | -1.38% | 100 | 28 | Township |
| Liberty | 7 | 2,307 | 2,322 | 0.04% | 66 | 35 | Township |
| Liberty | 8 | 2,229 | 2,207 | 0.41% | 48 | 47 | Township |
| Liberty | 9 | 2,067 | 2,026 | 0.44% | 62 | 33 | Township |
| Liberty | 10 | 1,965 | 2,043 | -0.81% | 2,497 | 1 | Town |
| Liberty | 11 | 1,955 | 1,960 | 0.46% | 91 | 22 | Township |
| Liberty | 12 | 1,761 | 1,764 | 0.57% | 75 | 24 | Township |
| Liberty | 13 | 1,499 | 1,551 | -1.51% | 32 | 47 | Township |
| Liberty | 14 | 1,433 | 1,424 | 0.14% | 35 | 41 | Township |
| Union | 15 | 1,407 | 1,463 | -1.26% | 46 | 30 | Township |
| Liberty | 16 | 1,110 | 1,069 | 0.73% | 31 | 36 | Township |
| Liberty | 17 | 1,008 | 1,046 | -1.18% | 43 | 24 | Township |
| Liberty | 18 | 896 | 896 | 0.11% | 21 | 42 | Township |
| Liberty | 19 | 877 | 881 | -0.57% | 20 | 44 | Township |
| Brownsville | 20 | 696 | 725 | -1.14% | 25 | 28 | Township |
| Liberty | 21 | 647 | 623 | 0.78% | 17 | 39 | Township |
| West College Corner | 22 | 508 | 526 | -0.78% | 1,903 | 0 | Town |
| Harrison | 23 | 398 | 417 | -1.49% | 13 | 30 | Township |
| Liberty | 24 | 390 | 381 | 0.52% | 22 | 17 | Township |
| Spring Grove | 25 | 331 | 333 | -0.3% | 1,089 | 0 | Town |
| Whitewater | 26 | 71 | 71 | 0% | 899 | 0 | Town |
Liberty is the largest city in Union County, with a population of 11,128.
Whitewater is the smallest city in Union County, with a population of 71.
There are 26 cities in Union County, Indiana.