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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 24 | Suffolk County | New York | 2362.20 | 912.05 | 1493350 | 632 | 1637 | 1494388 | 633 | 1638 | 1498816 | 635 | 1643 | 4428 | 0.30 | Riverhead |
| 25 | Sacramento County | California | 2498.42 | 964.64 | 1418788 | 568 | 1471 | 1422094 | 569 | 1474 | 1436105 | 575 | 1489 | 14011 | 0.99 | Sacramento |
| 26 | Bronx County | New York | 109.03 | 42.10 | 1385108 | 12704 | 32903 | 1387159 | 12723 | 32952 | 1392002 | 12767 | 33067 | 4843 | 0.35 | Bronx |
| 27 | Nassau County | New York | 737.41 | 284.72 | 1339532 | 1817 | 4705 | 1341033 | 1819 | 4710 | 1344436 | 1823 | 4722 | 3403 | 0.25 | Mineola |
| 28 | Palm Beach County | Florida | 5101.66 | 1969.76 | 1320134 | 259 | 670 | 1322924 | 259 | 672 | 1335187 | 262 | 678 | 12263 | 0.93 | West Palm Beach |
| 29 | Cuyahoga County | Ohio | 1184.12 | 457.19 | 1280122 | 1081 | 2800 | 1278000 | 1079 | 2795 | 1270294 | 1073 | 2778 | -7706 | -0.60 | Cleveland |