| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Hillsboro | 1 | 111,820 | 107,080 | 0.62% | 4,229 | 26 | City |
| Beaverton | 2 | 97,922 | 97,594 | 0.08% | 4,986 | 20 | City |
| Tigard | 3 | 58,434 | 55,025 | 0.87% | 4,581 | 13 | City |
| Tualatin | 4 | 28,152 | 27,878 | 0.05% | 3,352 | 8 | City |
| Forest Grove | 5 | 27,470 | 26,066 | 0.8% | 4,663 | 6 | City |
| Sherwood | 6 | 20,511 | 20,450 | -0.12% | 4,191 | 5 | City |
| Cornelius | 7 | 16,137 | 12,821 | 3.98% | 6,899 | 2 | City |
| King City | 8 | 4,995 | 5,175 | -0.97% | 6,159 | 1 | City |
| North Plains | 9 | 3,406 | 3,443 | -0.23% | 3,125 | 1 | City |
| Durham | 10 | 1,886 | 1,950 | -0.95% | 4,600 | 0 | City |
| Banks | 11 | 1,816 | 1,837 | -0.27% | 2,383 | 1 | City |
| Gaston | 12 | 670 | 679 | -0.3% | 1,965 | 0 | City |
Hillsboro is the largest city in Washington County, with a population of 111,820.
Gaston is the smallest city in Washington County, with a population of 670.
There are 12 cities in Washington County, Oregon.