| Population Change 1990-2000 | ||||
| County or Municipality | 2000 Population | 1990 Population | Population Change | Percent Change |
| Pennsylvania | 12 281 054 | 11 881 643 | 399 411 | 3.4 |
| Washington County | 202 897 | 204 584 | - 1 687 | - 0.8 |
| Allenport borough | 549 | 595 | - 46 | - 7.7 |
| Amwell township | 3 960 | 4 176 | - 216 | - 5.2 |
| Beallsville borough | 511 | 530 | - 19 | - 3.6 |
| Bentleyville borough | 2 502 | 2 673 | - 171 | - 6.4 |
| Blaine township | 597 | 682 | - 85 | - 12.5 |
| Buffalo township | 2 100 | 2 148 | - 48 | - 2.2 |
| Burgettstown borough | 1 576 | 1 634 | - 58 | - 3.5 |
| California borough | 5 274 | 5 748 | - 474 | - 8.2 |
| Canonsburg borough | 8 607 | 9 200 | - 593 | - 6.4 |
| Canton township | 8 826 | 9 256 | - 430 | - 4.6 |
| Carroll township | 5 677 | 6 210 | - 533 | - 8.6 |
| Cecil township | 9 756 | 8 948 | 808 | 9.0 |
| Centerville borough | 3 390 | 3 842 | - 452 | - 11.8 |
| Charleroi borough | 4 871 | 5 014 | - 143 | - 2.9 |
| Chartiers township | 7 154 | 7 603 | - 449 | - 5.9 |
| Claysville borough | 724 | 962 | - 238 | - 24.7 |
| Coal Center borough | 134 | 184 | - 50 | - 27.2 |
| Cokeburg borough | 705 | 724 | - 19 | - 2.6 |
| Cross Creek township | 1 685 | 1 727 | - 42 | - 2.4 |
| Deemston borough | 809 | 770 | 39 | 5.1 |
| Donegal township | 2 428 | 2 347 | 81 | 3.5 |
| Donora borough | 5 653 | 5 928 | - 275 | - 4.6 |
| Dunlevy borough | 397 | 417 | - 20 | - 4.8 |
| East Bethlehem township | 2 524 | 2 799 | - 275 | - 9.8 |
| East Finley township | 1 489 | 1 479 | 10 | 0.7 |
| East Washington borough | 1 930 | 2 126 | - 196 | - 9.2 |
| Elco borough | 362 | 373 | - 11 | - 2.9 |
| Ellsworth borough | 1 083 | 1 048 | 35 | 3.3 |
| Fallowfield township | 4 461 | 4 972 | - 511 | - 10.3 |
| Finleyville borough | 459 | 446 | 13 | 2.9 |
| Green Hills borough | 18 | 21 | - 3 | - 14.3 |
| Hanover township | 2 795 | 2 883 | - 88 | - 3.1 |
| Hopewell township | 992 | 942 | 50 | 5.3 |
| Houston borough | 1 314 | 1 445 | - 131 | - 9.1 |
| Independence township | 1 676 | 1 868 | - 192 | - 10.3 |
| Jefferson township | 1 218 | 1 212 | 6 | 0.5 |
| Long Branch borough | 539 | 482 | 57 | 11.8 |
| McDonald borough | 1 866 | 1 809 | 57 | 3.2 |
| Marianna borough | 626 | 616 | 10 | 1.6 |
| Midway borough | 982 | 1 043 | - 61 | - 5.8 |
| Monongahela city | 4 761 | 4 928 | - 167 | - 3.4 |
| Morris township | 1 272 | 1 145 | 127 | 11.1 |
| Mount Pleasant township | 3 422 | 3 555 | - 133 | - 3.7 |
| New Eagle borough | 2 262 | 2 172 | 90 | 4.1 |
| North Bethlehem township | 1 746 | 1 864 | - 118 | - 6.3 |
| North Charleroi borough | 1 409 | 1 562 | - 153 | - 9.8 |
| North Franklin township | 4 818 | 4 997 | - 179 | - 3.6 |
| North Strabane township | 10 057 | 8 157 | 1 900 | 23.3 |
| Nottingham township | 2 522 | 2 303 | 219 | 9.5 |
| Peters township | 17 566 | 14 467 | 3 099 | 21.4 |
| Robinson township | 2 193 | 2 160 | 33 | 1.5 |
| Roscoe borough | 848 | 872 | - 24 | - 2.8 |
| Smith township | 4 567 | 4 844 | - 277 | - 5.7 |
| Somerset township | 2 701 | 2 947 | - 246 | - 8.3 |
| South Franklin township | 3 796 | 3 665 | 131 | 3.6 |
| South Strabane township | 7 987 | 7 676 | 311 | 4.1 |
| Speers borough | 1 241 | 1 284 | - 43 | - 3.3 |
| Stockdale borough | 555 | 630 | - 75 | - 11.9 |
| Twilight borough | 241 | 252 | - 11 | - 4.4 |
| Union township | 5 599 | 6 322 | - 723 | - 11.4 |
| Washington city | 15 268 | 15 864 | - 596 | - 3.8 |
| West Alexander borough | 320 | 301 | 19 | 6.3 |
| West Bethlehem township | 1 432 | 1 609 | - 177 | - 11.0 |
| West Brownsville borough | 1 075 | 1 170 | - 95 | - 8.1 |
| West Finley township | 951 | 972 | - 21 | - 2.2 |
| West Middletown borough | 144 | 166 | - 22 | - 13.3 |
| West Pike Run township | 1 925 | 1 818 | 107 | 5.9 |
| Source: U.S. Census Bureau | Prepared by: Pennsylvania State Data Center | |||