List of the most populous counties in the United States List of the most populous US counties sorted according to the mid-2011 official estimate by the USCB List of the most populous US counties sorted according to the mid-2011 official estimate by the USCB

| Rank_ | County | State_ | Land_area_km² | Land_area_mi² | April_1_2010_census | April_1_2010_density_km² | April_1_2010_density_mi² | July_1_2010_official_estimate | July_1_2010_density_km² | July_1_2010_density_mi² | July_1_2011_official_estimate | July_1_2011_density_km² | July_1_2011_density_mi² | Mid_2010_to_mid_2011_change_Absolute | Mid_2010_to_mid_2011_change_ | County_seat_or_courthouse |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 78 | Jefferson County | Kentucky | 985.27 | 380.42 | 741096 | 752 | 1948 | 742176 | 753 | 1951 | 746906 | 758 | 1963 | 4730 | 0.64 | Louisville |
| 79 | Monroe County | New York | 1702.15 | 657.21 | 744344 | 437 | 1133 | 744635 | 437 | 1133 | 745625 | 438 | 1135 | 990 | 0.13 | Rochester |
| 80 | Oklahoma County | Oklahoma | 1835.83 | 708.82 | 718633 | 391 | 1014 | 721298 | 393 | 1018 | 732371 | 399 | 1033 | 11073 | 1.54 | Oklahoma City |
| 81 | Suffolk County | Massachusetts | 150.62 | 58.15 | 722023 | 4794 | 12416 | 722731 | 4798 | 12428 | 730932 | 4853 | 12569 | 8201 | 1.13 | Boston |
| 82 | San Mateo County | California | 1161.37 | 448.41 | 718451 | 619 | 1602 | 719604 | 620 | 1605 | 727209 | 626 | 1622 | 7605 | 1.06 | Redwood City |
| 83 | Snohomish County | Washington | 5406.01 | 2087.27 | 713335 | 132 | 342 | 715358 | 132 | 343 | 722400 | 134 | 346 | 7042 | 0.98 | Everett |