21 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| East Buffalo | 1 | 7,452 | 7,395 | -0.17% | 489 | 15 | Township |
| Lewisburg | 2 | 5,277 | 5,156 | 0.46% | 5,423 | 1 | Borough |
| White Deer | 3 | 4,353 | 4,404 | -0.48% | 94 | 46 | Township |
| Kelly | 4 | 4,038 | 4,001 | 5.62% | 235 | 17 | Township |
| Gregg | 5 | 3,583 | 4,092 | -4.99% | 241 | 15 | Township |
| Buffalo | 6 | 3,562 | 3,588 | -0.25% | 118 | 30 | Township |
| Mifflinburg | 7 | 3,467 | 3,482 | -0.09% | 1,922 | 2 | Borough |
| Winfield | 8 | 3,293 | 3,362 | -0.06% | 138 | 24 | Township |
| West Buffalo | 9 | 2,842 | 2,883 | -0.46% | 74 | 38 | Township |
| Linntown | 10 | 1,736 | 1,799 | 0.93% | 2,327 | 1 | CDP |
| Hartley | 11 | 1,715 | 1,749 | -0.58% | 22 | 80 | Township |
| Union | 12 | 1,608 | 1,630 | -0.31% | 144 | 11 | Township |
| Lewis | 13 | 1,470 | 1,482 | 0.07% | 38 | 39 | Township |
| Winfield | 14 | 836 | 872 | -0.48% | 291 | 3 | CDP |
| New Columbia | 15 | 816 | 1,164 | -1.57% | 423 | 2 | CDP |
| New Berlin | 16 | 774 | 794 | -0.51% | 1,995 | 0 | Borough |
| West Milton | 17 | 596 | 1,152 | -3.25% | 718 | 1 | CDP |
| Laurelton | 18 | 402 | 336 | 3.08% | 705 | 1 | CDP |
| Allenwood | 19 | 312 | 195 | -0.32% | 531 | 1 | CDP |
| Hartleton | 20 | 247 | 251 | -0.4% | 272 | 1 | Borough |
| Vicksburg | 21 | 136 | 283 | -5.56% | 282 | 0 | CDP |
East Buffalo is the largest city in Union County, with a population of 7,452.
Vicksburg is the smallest city in Union County, with a population of 136.
There are 21 cities in Union County, Pennsylvania.