13 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Wilmington | 1 | 73,759 | 70,842 | 0.8% | 6,770 | 11 | City |
| Newark | 2 | 30,483 | 30,598 | -0.08% | 3,218 | 9 | City |
| Middletown | 3 | 26,259 | 23,392 | 2.23% | 1,944 | 14 | Town |
| Elsmere | 4 | 6,236 | 6,223 | 0.03% | 6,318 | 1 | Town |
| New Castle | 5 | 5,758 | 5,543 | 0.75% | 1,656 | 3 | City |
| Townsend | 6 | 2,990 | 2,743 | 1.67% | 2,821 | 1 | Town |
| Delaware City | 7 | 1,945 | 1,883 | 0.62% | 1,045 | 2 | City |
| Bellefonte | 8 | 1,255 | 1,225 | 0.48% | 7,090 | 0 | Town |
| Newport | 9 | 909 | 908 | 0% | 1,946 | 0 | Town |
| Arden | 10 | 438 | 430 | 0.23% | 1,759 | 0 | Village |
| Odessa | 11 | 366 | 365 | 0% | 722 | 1 | Town |
| Ardentown | 12 | 259 | 254 | 0.39% | 1,107 | 0 | Village |
| Ardencroft | 13 | 235 | 228 | 0.43% | 2,640 | 0 | Village |
Wilmington is the largest city in New Castle County, with a population of 73,759.
Ardencroft is the smallest city in New Castle County, with a population of 235.
There are 13 cities in New Castle County, Delaware.