23 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Elizabeth | 1 | 137,397 | 136,677 | 0.57% | 11,153 | 12.32 | City |
| Union township | 2 | 60,934 | 59,826 | 0.21% | 6,732 | 9.05 | Township |
| Plainfield | 3 | 55,008 | 54,340 | 0.31% | 9,231 | 5.96 | City |
| Linden | 4 | 45,244 | 43,540 | 1.45% | 4,233 | 10.69 | City |
| Westfield | 5 | 30,511 | 30,895 | -0.08% | 4,538 | 6.72 | Town |
| Rahway | 6 | 30,099 | 29,671 | 0.48% | 7,726 | 3.90 | City |
| Scotch Plains township | 7 | 24,344 | 24,852 | -0.18% | 2,699 | 9.02 | Township |
| Cranford township | 8 | 23,697 | 23,817 | -0.09% | 4,901 | 4.83 | Township |
| Roselle borough | 9 | 22,410 | 22,604 | 0.15% | 8,505 | 2.63 | Borough |
| Summit | 10 | 22,390 | 22,638 | 0.1% | 3,735 | 5.99 | City |
| Hillside township | 11 | 21,939 | 22,344 | -0.12% | 7,929 | 2.77 | Township |
| Springfield township | 12 | 16,822 | 17,106 | -0.07% | 3,262 | 5.16 | Township |
| Clark township | 13 | 15,803 | 15,472 | 1.19% | 3,698 | 4.27 | Township |
| Roselle Park borough | 14 | 13,894 | 14,002 | -0.14% | 11,342 | 1.23 | Borough |
| Berkeley Heights township | 15 | 13,759 | 13,223 | 1.72% | 2,212 | 6.22 | Township |
| New Providence borough | 16 | 13,442 | 13,680 | -0.17% | 3,645 | 3.69 | Borough |
| Kenilworth borough | 17 | 8,237 | 8,393 | -0.13% | 3,837 | 2.15 | Borough |
| Fanwood borough | 18 | 7,659 | 7,748 | 0.03% | 5,716 | 1.34 | Borough |
| Mountainside borough | 19 | 6,898 | 6,988 | -0.19% | 1,726 | 4 | Borough |
| Union township | 20 | 6,489 | 6,494 | 0.76% | 346 | 18.77 | Township |
| Garwood borough | 21 | 5,469 | 4,429 | 2.9% | 8,414 | 0.65 | Borough |
| Springfield township | 22 | 3,263 | 3,238 | 0.18% | 111 | 29.51 | Township |
| Winfield township | 23 | 1,388 | 1,415 | -0.14% | 7,669 | 0.18 | Township |
Elizabeth is the largest city in Union County, with a population of 137,397.
Winfield township is the smallest city in Union County.
There are 23 cities in Union County, New Jersey.