| County | FIPS Code | County Seat | Established in | Population | Area (sq mi) | Area (km2) |
|---|---|---|---|---|---|---|
| Adams County | 1 | Gettysburg | 1800 | 101,407 | 522 | 1,352 |
| Allegheny County | 3 | Pittsburgh | 1788 | 1,223,348 | 745 | 1,930 |
| Armstrong County | 5 | Kittanning | 1800 | 68,941 | 664 | 1,720 |
| Beaver County | 7 | Beaver | 1800 | 170,539 | 444 | 1,150 |
| Bedford County | 9 | Bedford | 1771 | 49,762 | 1,015 | 2,629 |
| Berks County | 11 | Reading | 1752 | 411,442 | 866 | 2,243 |
| Blair County | 13 | Hollidaysburg | 1846 | 127,089 | 527 | 1,365 |
| Bradford County | 15 | Towanda | 1810 | 62,622 | 1,161 | 3,007 |
| Bucks County | 17 | Doylestown | 1682 | 625,249 | 622 | 1,611 |
| Butler County | 19 | Butler | 1800 | 183,862 | 795 | 2,059 |
| Cambria County | 21 | Ebensburg | 1804 | 143,679 | 693 | 1,795 |
| Cameron County | 23 | Emporium | 1860 | 5,085 | 399 | 1,033 |
| Carbon County | 25 | Jim Thorpe | 1843 | 65,249 | 387 | 1,002 |
| Centre County | 27 | Bellefonte | 1800 | 153,990 | 1,112 | 2,880 |
| Chester County | 29 | West Chester | 1682 | 498,886 | 760 | 1,968 |
| Clarion County | 31 | Clarion | 1839 | 39,988 | 609 | 1,577 |
| Clearfield County | 33 | Clearfield | 1804 | 81,642 | 1,154 | 2,989 |
| Clinton County | 35 | Lock Haven | 1839 | 39,238 | 898 | 2,326 |
| Columbia County | 37 | Bloomsburg | 1813 | 67,295 | 490 | 1,269 |
| Crawford County | 39 | Meadville | 1800 | 88,765 | 1,038 | 2,688 |
| Cumberland County | 41 | Carlisle | 1750 | 235,406 | 551 | 1,427 |
| Dauphin County | 43 | Harrisburg | 1785 | 268,100 | 558 | 1,445 |
| Delaware County | 45 | Media | 1789 | 558,979 | 191 | 495 |
| Elk County | 47 | Ridgway | 1843 | 31,946 | 832 | 2,155 |
| Erie County | 49 | Erie | 1800 | 280,566 | 799 | 2,069 |
| Fayette County | 51 | Uniontown | 1783 | 136,606 | 798 | 2,067 |
| Forest County | 53 | Tionesta | 1848 | 7,716 | 431 | 1,116 |
| Franklin County | 55 | Chambersburg | 1784 | 149,618 | 771 | 1,997 |
| Fulton County | 57 | McConnellsburg | 1850 | 14,845 | 438 | 1,134 |
| Greene County | 59 | Waynesburg | 1796 | 38,686 | 578 | 1,497 |
| Huntingdon County | 61 | Huntingdon | 1787 | 45,913 | 889 | 2,302 |
| Indiana County | 63 | Indiana | 1803 | 88,880 | 834 | 2,160 |
| Jefferson County | 65 | Brookville | 1804 | 45,200 | 657 | 1,702 |
| Juniata County | 67 | Mifflintown | 1831 | 24,636 | 394 | 1,020 |
| Lackawanna County | 69 | Scranton | 1878 | 214,437 | 465 | 1,204 |
| Lancaster County | 71 | Lancaster | 1729 | 519,445 | 984 | 2,549 |
| Lawrence County | 73 | New Castle | 1849 | 91,108 | 363 | 940 |
| Lebanon County | 75 | Lebanon | 1813 | 133,568 | 363 | 940 |
| Lehigh County | 77 | Allentown | 1812 | 349,497 | 349 | 904 |
| Luzerne County | 79 | Wilkes-Barre | 1786 | 320,918 | 907 | 2,349 |
| Lycoming County | 81 | Williamsport | 1795 | 116,111 | 1,244 | 3,222 |
| McKean County | 83 | Smethport | 1804 | 43,450 | 984 | 2,549 |
| Mercer County | 85 | Mercer | 1800 | 116,638 | 683 | 1,769 |
| Mifflin County | 87 | Lewistown | 1789 | 46,682 | 415 | 1,075 |
| Monroe County | 89 | Stroudsburg | 1836 | 169,842 | 617 | 1,598 |
| Montgomery County | 91 | Norristown | 1784 | 799,874 | 487 | 1,261 |
| Montour County | 93 | Danville | 1850 | 18,267 | 132 | 342 |
| Northampton County | 95 | Easton | 1752 | 297,735 | 377 | 976 |
| Northumberland County | 97 | Sunbury | 1772 | 94,528 | 477 | 1,235 |
| Perry County | 99 | New Bloomfield | 1820 | 45,969 | 556 | 1,440 |
| Philadelphia County | 101 | Philadelphia | 1682 | 1,526,006 | 143 | 370 |
| Pike County | 103 | Milford | 1814 | 57,369 | 567 | 1,469 |
| Potter County | 105 | Coudersport | 1804 | 17,457 | 1,081 | 2,800 |
| Schuylkill County | 107 | Pottsville | 1811 | 148,289 | 778 | 2,015 |
| Snyder County | 109 | Middleburg | 1855 | 39,702 | 332 | 860 |
| Somerset County | 111 | Somerset | 1795 | 77,742 | 1,081 | 2,800 |
| Sullivan County | 113 | Laporte | 1847 | 6,428 | 452 | 1,171 |
| Susquehanna County | 115 | Montrose | 1810 | 43,356 | 832 | 2,155 |
| Tioga County | 117 | Wellsboro | 1804 | 41,981 | 1,137 | 2,945 |
| Union County | 119 | Lewisburg | 1813 | 44,947 | 317 | 821 |
| Venango County | 121 | Franklin | 1800 | 54,984 | 683 | 1,769 |
| Warren County | 123 | Warren | 1800 | 41,815 | 898 | 2,326 |
| Washington County | 125 | Washington | 1781 | 207,820 | 861 | 2,230 |
| Wayne County | 127 | Honesdale | 1798 | 52,822 | 751 | 1,945 |
| Westmoreland County | 129 | Greensburg | 1773 | 365,169 | 1,036 | 2,683 |
| Wyoming County | 131 | Tunkhannock | 1842 | 28,276 | 405 | 1,049 |
| York County | 133 | York | 1749 | 434,972 | 910 | 2,357 |
