| County | FIPS code | County seat | Established | Origin | Etymology | Population | Area | Map |
| Adams County | 001 | Gettysburg | 1800 | Parts of York County | John Adams, second U.S. President | 107,914 | 522 sq mi(1,352 km2) | |
| Allegheny County | 003 | Pittsburgh | 1788 | Parts of Washington and Westmoreland Counties | Delaware word for the Allegheny River, which possibly translates to "beautiful river" | 1,231,814 | 745 sq mi(1,930 km2) | |
| Armstrong County | 005 | Kittanning | 1800 | Parts of Allegheny, Lycoming, and Westmoreland Counties | John Armstrong, Revolutionary War general | 63,679 | 664 sq mi(1,720 km2) | |
| Beaver County | 007 | Beaver | 1800 | Parts of Allegheny and Washington Counties | Beaver River, itself named for the eponymous animal that was sighted along its banks | 165,540 | 444 sq mi(1,150 km2) | |
| Bedford County | 009 | Bedford | 1771 | Parts of Cumberland County | Fort Bedford, which is named for John Russell, 4th Duke of Bedford | 47,643 | 1,015 sq mi(2,629 km2) | |
| Berks County | 011 | Reading | 1752 | Parts of Chester, Lancaster and Philadelphia Counties | The English county of Berkshire | 439,117 | 866 sq mi(2,243 km2) | |
| Blair County | 013 | Hollidaysburg | 1846 | Parts of Huntingdon and Bedford Counties | John Blair (1766-1832) a local pioneer farmer, merchant, and public servant | 120,269 | 527 sq mi(1,365 km2) | |
| Bradford County | 015 | Towanda | 1810 | Parts of Luzerne and Lycoming Counties; originally called Ontario County, renamed as Bradford County in 1812. | William Bradford, second U.S. Attorney General | 59,699 | 1,161 sq mi(3,007 km2) | |
| Bucks County | 017 | Doylestown | 1682 | One of the original counties at the formation of Pennsylvania | The English county of Buckinghamshire | 650,131 | 622 sq mi(1,611 km2) | |
| Butler County | 019 | Butler | 1800 | Parts of Allegheny County | Richard Butler, Revolutionary War general | 199,341 | 795 sq mi(2,059 km2) | |
| Cambria County | 021 | Ebensburg | 1804 | Parts of Somerset and Huntingdon Counties | Cambria, the traditional name for Wales | 130,108 | 693 sq mi(1,795 km2) | |
| Cameron County | 023 | Emporium | 1860 | Parts of Clinton, Elk, McKean, and Potter Counties | Simon Cameron, U.S. Senator from Pennsylvania | 4,348 | 399 sq mi(1,033 km2) | |
| Carbon County | 025 | Jim Thorpe | 1843 | Parts of Monroe and Northampton Counties | Carbon, the element found in coal | 65,743 | 387 sq mi(1,002 km2) | |
| Centre County | 027 | Bellefonte | 1800 | Parts of Lycoming, Mifflin, Northumberland, and Huntingdon Counties | Centre Furnace, the first industrial facility in the area | 159,805 | 1,112 sq mi(2,880 km2) | |
| Chester County | 029 | West Chester | 1682 | One of the original counties at the formation of Pennsylvania | The English city of Chester in the county of Cheshire | 560,745 | 760 sq mi(1,968 km2) | |
| Clarion County | 031 | Clarion | 1839 | Parts of Venango and Armstrong Counties | Clarion River, itself so named for its clarity | 36,855 | 609 sq mi(1,577 km2) | |
| Clearfield County | 033 | Clearfield | 1804 | Parts of Lycoming and Huntingdon Counties; Clearfield functioned as a part of Centre County for judiciary purposes until 1822. | The cleared fields from logging in the area | 78,132 | 1,154 sq mi(2,989 km2) | |
| Clinton County | 035 | Lock Haven | 1839 | Parts of Lycoming and Centre Counties | DeWitt Clinton, New York Governor and prominent statesman | 37,865 | 898 sq mi(2,326 km2) | |
| Columbia County | 037 | Bloomsburg | 1813 | Parts of Northumberland and Luzerne Counties | Columbia, the first popular and poetic name for the United States | 66,012 | 490 sq mi(1,269 km2) | |
| Crawford County | 039 | Meadville | 1800 | Parts of Allegheny County | William Crawford, surveyor who helped to open trans-Appalachian lands to settlement | 82,089 | 1,038 sq mi(2,688 km2) | |
| Cumberland County | 041 | Carlisle | 1750 | Parts of Lancaster County | The historic English county of Cumberland | 275,516 | 551 sq mi(1,427 km2) | |
| Dauphin County | 043 | Harrisburg | 1785 | Parts of Lancaster County | Louis-Joseph, Dauphin of France | 293,029 | 558 sq mi(1,445 km2) | |
| Delaware County | 045 | Media | 1789 | Parts of Chester County | Delaware River, itself named for Lord De La Warr | 584,882 | 191 sq mi(495 km2) | |
| Elk County | 047 | Ridgway | 1843 | Parts of Jefferson, McKean, and Clearfield Counties | Elk, which inhabit the forested county | 30,124 | 832 sq mi(2,155 km2) | |
| Erie County | 049 | Erie | 1800 | Parts of Allegheny County; attached to Crawford County until 1803. | Lake Erie | 267,750 | 799 sq mi(2,069 km2) | |
| Fayette County | 051 | Uniontown | 1783 | Parts of Westmoreland County | The Marquis de Lafayette, French-born Revolutionary War general | 123,941 | 798 sq mi(2,067 km2) | |
| Forest County | 053 | Tionesta | 1848 | Parts of Jefferson County; attached to Jefferson County until 1857. | Chief natural feature | 6,601 | 431 sq mi(1,116 km2) | |
| Franklin County | 055 | Chambersburg | 1784 | Parts of Cumberland County | Benjamin Franklin, key Founding Father of the United States | 159,285 | 771 sq mi(1,997 km2) | |
| Fulton County | 057 | McConnellsburg | 1850 | Parts of Bedford County | Robert Fulton, inventor of the steamboat | 14,452 | 438 sq mi(1,134 km2) | |
| Greene County | 059 | Waynesburg | 1796 | Parts of Washington County | Nathanael Greene, Revolutionary War general | 33,960 | 578 sq mi(1,497 km2) | |
| Huntingdon County | 061 | Huntingdon | 1787 | Parts of Bedford County | The historic English county of Huntingdonshire | 43,359 | 889 sq mi(2,302 km2) | |
| Indiana County | 063 | Indiana | 1803 | Parts of Lycoming and Westmoreland Counties; it was attached to Westmoreland County until 1806. | From the Indiana Grant of 1768, that the Iroquois Six Nations were forced to make to "suffering traders" under the Fort Stanwix Treaty of 1768. | 82,953 | 834 sq mi(2,160 km2) | |
| Jefferson County | 065 | Brookville | 1804 | Parts of Lycoming County Attached to Westmoreland County until 1806 and to Indiana County until 1830. | Thomas Jefferson, third U.S. President | 43,367 | 657 sq mi(1,702 km2) | |
| Juniata County | 067 | Mifflintown | 1831 | Parts of Mifflin County | Juniata River, itself named for the Iroquoian word Onayutta, meaning "Standing Stone" | 23,395 | 394 sq mi(1,020 km2) | |
| Lackawanna County | 069 | Scranton | 1878 | Parts of Luzerne County | Lackawanna River, itself named for the Delaware word meaning "stream that forks" | 216,859 | 465 sq mi(1,204 km2) | |
| Lancaster County | 071 | Lancaster | 1729 | Parts of Chester County | The English city of Lancaster | 563,293 | 984 sq mi(2,549 km2) | |
| Lawrence County | 073 | New Castle | 1849 | Parts of Beaver and Mercer Counties | USS Lawrence (1813), itself named in honor of James Lawrence, War of 1812 captain | 84,233 | 363 sq mi(940 km2) | |
| Lebanon County | 075 | Lebanon | 1813 | Parts of Dauphin and Lancaster Counties | Lebanon, the Biblical term for "White Mountain", which references the piety of the county's Moravian founders | 145,319 | 363 sq mi(940 km2) | |
| Lehigh County | 077 | Allentown | 1812 | Parts of Northampton County | Lehigh River, from the Lenape Lechewuekink, "where there are forks". | 385,655 | 349 sq mi(904 km2) | |
| Luzerne County | 079 | Wilkes-Barre | 1786 | Parts of Northumberland County | Anne-César, Chevalier de la Luzerne, French ambassador to the U.S. who aided republican causes | 331,379 | 907 sq mi(2,349 km2) | |
| Lycoming County | 081 | Williamsport | 1795 | Parts of Northumberland County; originally called Jefferson County, ultimately was renamed as Lycoming County. | Lycoming Creek, itself named for the Delaware word iacomic meaning "great steam" | 113,236 | 1,244 sq mi(3,222 km2) | |
| McKean County | 083 | Smethport | 1804 | Parts of Lycoming County; Attached to Centre County until 1814 and to Lycoming County until 1826 for judicial and elective purposes. McKean was fully organized only in 1826. | Thomas McKean, second Governor of Pennsylvania | 39,478 | 984 sq mi(2,549 km2) | |
| Mercer County | 085 | Mercer | 1800 | Parts of Allegheny County | Hugh Mercer, Revolutionary War general | 108,140 | 683 sq mi(1,769 km2) | |
| Mifflin County | 087 | Lewistown | 1789 | Parts of Cumberland and Northumberland Counties | Thomas Mifflin, first Governor of Pennsylvania | 45,935 | 415 sq mi(1,075 km2) | |
| Monroe County | 089 | Stroudsburg | 1836 | Parts of Pike and Northampton Counties | James Monroe, fifth U.S President | 166,523 | 617 sq mi(1,598 km2) | |
| Montgomery County | 091 | Norristown | 1784 | Parts of Philadelphia County | The historic Welsh county of Montgomeryshire or Richard Montgomery, a general killed in the 1775 Battle of Quebec | 879,190 | 487 sq mi(1,261 km2) | |
| Montour County | 093 | Danville | 1850 | Parts of Columbia County | Madame Montour, colonial ambassador to the Native Americans | 18,115 | 132 sq mi(342 km2) | |
| Northampton County | 095 | Easton | 1752 | Parts of Bucks County | The English town of Northampton | 322,989 | 377 sq mi(976 km2) | |
| Northumberland County | 097 | Sunbury | 1772 | Parts of Lancaster, Berks, Bedford, Cumberland, and Northampton Counties | The English county of Northumberland | 90,027 | 477 sq mi(1,235 km2) | |
