42 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Elizabeth city | 1 | 137,397 | 136,677 | 0.57% | 11,153 | 12.32 | City |
| Elizabeth | 2 | 135,263 | 136,677 | -0.21% | 10,980 | 12.32 | City |
| Union township | 3 | 60,934 | 59,826 | 0.21% | 6,732 | 9.05 | Township |
| Plainfield city | 4 | 55,008 | 54,340 | 0.31% | 9,231 | 5.96 | City |
| Plainfield | 5 | 54,890 | 54,340 | 0.2% | 9,211 | 5.96 | City |
| Linden city | 6 | 45,244 | 43,540 | 1.45% | 4,233 | 10.69 | City |
| Linden | 7 | 44,223 | 43,540 | 0.31% | 4,138 | 10.69 | City |
| Westfield town | 8 | 30,511 | 30,895 | -0.08% | 4,538 | 6.72 | Town |
| Westfield | 9 | 30,335 | 30,895 | -0.37% | 4,511 | 6.72 | Town |
| Rahway city | 10 | 30,099 | 29,671 | 0.48% | 7,726 | 3.90 | City |
| Rahway | 11 | 29,907 | 29,671 | 0.16% | 7,676 | 3.90 | City |
| Scotch Plains township | 12 | 24,344 | 24,852 | -0.18% | 2,699 | 9.02 | Township |
| Cranford township | 13 | 23,697 | 23,817 | -0.09% | 4,901 | 4.83 | Township |
| Roselle borough | 14 | 22,410 | 22,604 | 0.15% | 8,505 | 2.63 | Borough |
| Summit city | 15 | 22,390 | 22,638 | 0.1% | 3,735 | 5.99 | City |
| Roselle | 16 | 22,167 | 22,604 | -0.39% | 8,413 | 2.63 | Borough |
| Summit | 17 | 22,148 | 22,638 | -0.44% | 3,695 | 5.99 | City |
| Hillside township | 18 | 21,939 | 22,344 | -0.12% | 7,929 | 2.77 | Township |
| Springfield township | 19 | 16,822 | 17,106 | -0.07% | 3,262 | 5.16 | Township |
| Clark township | 20 | 15,803 | 15,472 | 1.19% | 3,698 | 4.27 | Township |
| Roselle Park borough | 21 | 13,894 | 14,002 | -0.14% | 11,342 | 1.23 | Borough |
| Roselle Park | 22 | 13,885 | 14,002 | -0.17% | 11,335 | 1.23 | Borough |
| Berkeley Heights township | 23 | 13,759 | 13,223 | 1.72% | 2,212 | 6.22 | Township |
| New Providence borough | 24 | 13,442 | 13,680 | -0.17% | 3,645 | 3.69 | Borough |
| New Providence | 25 | 13,360 | 13,680 | -0.48% | 3,623 | 3.69 | Borough |
| Kenilworth borough | 26 | 8,237 | 8,393 | -0.13% | 3,837 | 2.15 | Borough |
| Kenilworth | 27 | 8,169 | 8,393 | -0.55% | 3,805 | 2.15 | Borough |
| Fanwood borough | 28 | 7,659 | 7,748 | 0.03% | 5,716 | 1.34 | Borough |
| Fanwood | 29 | 7,593 | 7,748 | -0.41% | 5,666 | 1.34 | Borough |
| Mountainside borough | 30 | 6,898 | 6,988 | -0.19% | 1,726 | 4 | Borough |
| Mountainside | 31 | 6,881 | 6,988 | -0.3% | 1,722 | 4 | Borough |
| Vauxhall | 32 | 6,664 | 5,677 | 7.14% | 11,314 | 0.59 | CDP |
| Union township | 33 | 6,489 | 6,494 | 0.76% | 346 | 18.77 | Township |
| Garwood | 34 | 5,649 | 4,429 | 4.51% | 8,691 | 0.65 | Borough |
| Garwood borough | 35 | 5,469 | 4,429 | 2.9% | 8,414 | 0.65 | Borough |
| Union | 36 | 3,718 | 2,591 | 7.15% | 14,580 | 0.26 | CDP |
| Springfield township | 37 | 3,263 | 3,238 | 0.18% | 111 | 29.51 | Township |
| Cranford | 38 | 2,160 | 2,008 | 7.14% | 6,409 | 0.34 | CDP |
| Springfield | 39 | 1,826 | 1,498 | 7.16% | 4,706 | 0.39 | CDP |
| Kean University | 40 | 1,640 | 1,187 | 7.12% | 5,616 | 0.29 | CDP |
| Winfield township | 41 | 1,388 | 1,415 | -0.14% | 7,669 | 0.18 | Township |
| Connecticut Farms | 42 | 629 | 566 | 7.16% | 6,837 | 0.09 | CDP |
Elizabeth city is the largest city in Union County, with a population of 137,397.
Connecticut Farms is the smallest city in Union County.
There are 42 cities in Union County, New Jersey.