| Adams County | 22 Jan 1800 | York County | In honor of John Adams (1735-1826); lawyer, a Founding Father, the first vice president, and second President of the United States (1797-1801) | Gettysburg | |
| Allegheny County | 24 Sep 1788 | Washington and Westmoreland Counties | Lenape word for the Allegheny River, which possibly translates to "beautiful river" | Pittsburgh | |
| Armstrong County | 12 Mar 1800 | Allegheny, Lycoming, and Westmoreland Counties | In honor of John Armstrong Sr. (1717-1795); a civil engineer, s a brigadier general in the Continental Army and as a major general in the Pennsylvania Militia during the American Revolutionary War. | Kittanning | |
| Beaver County | 12 Mar 1800 | Allegheny and Washington Counties | Beaver River, itself named for the eponymous animal that was sighted along its banks | Beaver | |
| Bedford County | 09 Mar 1771 | Cumberland County | In honor of Fort Bedford, which is named for John Russell, 4th Duke of Bedford | Bedford | |
| Berks County | 11 Mar 1752 | Chester, Lancaster, and Philadelphia Counties | English county of Berkshire | Reading | |
| Blair County | 26 Feb 1846 | Huntingdon and Bedford Counties | In honor of John Blair, Pennsylvania state legislator | Hollidaysburg | |
| Bradford County | 21 Feb 1810 | Luzerne and Lycoming Counties | In honor of William Bradford (1755-1795), a lawyer, judge, and second U.S. Attorney General | Towanda | |
| Bucks County | 01 Nov 1682 | One of the original counties at the formation of Pennsylvania | English county of Buckinghamshire | Doylestown | |
| Butler County | 12 Mar 1800 | Allegheny County | In honor of Richard Butler (1743-1791), a major general in the American Revolutionary War | Butler | |
| Carbon County | 26 Mar 1804 | Somerset and Huntingdon Counties | Cambria, the traditional name for Wales | Ebensburg | |
| Cambria County | 29 Mar 1860 | Clinton, Elk, McKean, and Potter Counties | In honor of Simon Cameron (1799-1889); an influential businessman and U.S. Senator from Pennsylvania | Emporium | |
| Centre County | 13 Mar 1843 | Monroe and Northampton Counties | Carbon, the element found in coal | Jim Thorpe | |
| Chester County | 13 Feb 1800 | Lycoming, Mifflin, Northumberland, and Huntingdon Counties | Centre Furnance, the first industrial facility in the area | Bellefonte | |
| Clarion County | 01 Nov 1682 | One of the original counties at the formation of Pennsylvania. | English city of Chester in the county of Cheshire | West Chester | |
| Clearfield County | 11 Mar 1839 | Venango and Armstrong Counties | Clarion River, itself so named for its clarity | Clarion | |
| Clinton County | 26 Mar 1804 | Lycoming and Huntingdon Counties | The cleared fields from logging in the area | Clearfield | |
| Columbia County | | Lycoming and Centre Counties | In honor of DeWitt Clinton (1769-1828), a New York Governor and prominant statesman | Lock Haven | |
| Crawford County | 22 Mar 1813 | Northumberland and Luzerne Counties | Columbia, the first popular and poetic name for the United States | Bloomsburg | |
| Cameron County | 12 Mar 1800 | Allegheny County | In honor of William Crawford (1722-1782), a surveyor who helped to open trans-Appalachian lands to settlement | Meadville | |
| Cumberland County | 27 Jan 1750 | Lancaster County | The historic English county of Cumberland | Carlisle | |
| Dauphin County | 04 Mar 1785 | Lancaster County | In honor of Louis-Joseph, Dauphin of France (1781-1789); the second child and elder son of King Louis XVI of France and Marie Antoinette | Harrisburg | |
| Delaware County | 06 Sep 1789 | Chester County | Delaware River, itself named for the Delaware tribe | Media | |
| Elk County | 18 Apr 1843 | Jefferson, McKean, and Clearfield Counties | Elk, which inhabit the forested county | Ridgway | |
| Erie County | 12 Mar 1800 | Allegheny County | Lake Erie | Erie | |
| Fayette County | 26 Sep 1783 | Westmoreland County | In honor of Marquis de Lafayette (1757-1834); a French aristocrat and major general in the American Revolutionary War | Uniontown | |
| Forest County | 01 Apr 1848 | Jefferson County | The large forests, its chief natural feature | Tionesta | Part of Venango County was added on October 31, 1866 |
| Franklin County | 09 Sep 1784 | Cumberland County | In honor of Benjamin Franklin (1705-1790); a Founding Father, a leading author, printer, political theorist, politician, freemason, postmaster, scientist, inventor, humorist, civic activist, statesman, and diplomat | Chambersburg | |
| Fulton County | 19 Apr 1851 | Bedford County | In honor of Robert Fulton (1765-1815), inventor of the steamboat | McConnellsburg | |
| Greene County | 09 Feb 1796 | Washington County | In honor of Nathanael Greene (1742-1786), a major general of the Continental Army in the American Revolutionary War | Waynesburg | |
| Huntingdon County | 20 Sep 1787 | Bedford County | The historic English county of Huntingdonshire | Huntingdon | |
| Indiana County | 30 Mar 1803 | Lycoming and Westmoreland Counties | Native Americans | Indiana | |
| Jefferson County | 26 Mar 1804 | Lycoming County | In honor of Thomas Jefferson (1743-1826); a statesman, diplomat, architect, Founding Father, and third president of the United States (1801-1809) | Brookville | |
| Juniata County | 02 Mar 1831 | Mifflin County | Iroquoian word "Onayutta", meaning "Standing Stone" | Mifflintown | |
| Lackawanna County | 13 Aug 1878 | Luzerne County | Lackawanna River, itself named for the Lenape word meaning "stream that forks" | Scranton | |
| Lancaster County | 10 May 1729 | Chester County | English city of Lancaster. | Lancaster | |
| Lawrence County | 20 Mar 1849 | Beaver and Mercer Counties | In honor of James Lawrence (1781-1813), a naval captain during the War of 1812. He is probably best known today for his last words or "dying command" "Don't give up the ship!" | New Castle | |
| Lebanon County | 16 Feb 1813 | Dauphin and Lancaster Counties | The Biblical term for "White Mountain", which references the piety of the county's Moravian founders | Lebanon | |
| Lehigh County | 06 Mar 1812 | Northampton County | Lehigh River | Allentown | |
| Luzerne County | 25 Sep 1786 | Northumberland County | In honor of Anne-César, Chevalier de la Luzerne (1741-1791), the French ambassador to the U.S. who aided republican causes | Wilkes-Barre | |
| Lycoming County | 13 Apr 1795 | Northumberland County | Lycoming Creek, itself named for the Lenape word "iacomic", meaning "great steam" | Williamsport | |
| McKean County | 26 Mar 1804 | Lycoming County | In honor of Thomas McKean (1734-1817), the second Governor of Pennsylvania | Smethport | |
| Mercer County | 12 Mar 1800 | Allegheny County | In honor of Hugh Mercer (1726-1777); a Scottish soldier, physician, and brigadier general in the Continental Army during the American Revolutionary War. | Mercer | |
| Mifflin County | 19 Sep 1789 | Cumberland and Northumberland Counties | In honor of Thomas Mifflin (1744-1800), the first Governor of Pennsylvania | Lewistown | |
| Monroe County | 01 Apr 1836 | Pike and Northampton Counties | In honor of James Monroe (1758-1831); a Founding Father who served as the fifth president of the United States (1817-1825) | Stroudsburg | |
| Montour County | 03 May 1850 | Columbia County | in honor of Madame Montour (1667 or c. 1685-c. 1753), colonial ambassador to the Native Americans | Danville | |
| Montgomery County | 10 Sep 1784 | Philadelphia County | The historic Welsh county of Montgomeryshire or Richard Montgomery (1738-1775), a major general in the American Revolutionary War. He was killed in the 1775 Battle of Quebec | Norristown | |
| Northampton County | 11 Mar 1752 | Bucks County | English city of Northampton | Easton | |
| Northumberland County | 21 Mar 1772 | Lancaster, Berks, Bedford, Cumberland, and Northampton Counties | The English county of Northumberland | Sunbury | |
| Perry County | 22 Mar 1820 | Cumberland County | In honor of Oliver Hazard Perry (1785-1819); a naval commodore in the War of 1812 | New Bloomfield | |
| Philadelphia County | 15 Nov 1682 | One of the three original counties | The "Brotherly love" from Greek philos ("love") and adelphos ("brother") | Philadelphia | |
| Pike County | 26 Mar 1814 | Wayne County | In honor of Zebulon Pike (1779-1813), an American brigadier general and explorer for whom Pikes Peak in Colorado was renamed (from El Capitan) | Milford | |
| Potter County | 26 Mar 1804 | Lycoming County | In honor of James Potter (1729-1789), a farmer, and politician from Colonial- and Revolutionary-era Pennsylvania and a brigadier general of Pennsylvania militia during the American Revolutionary War | Coudersport | |
| Schuylkill County | 01 Mar 1811 | Berks and Northampton Counties | Schuylkill River, itself a Dutch corruption of a Lenape word possibly meaning "hidden river" | Pottsville | Parts of Columbia County and Luzerne County was added on March 3, 1818 |
| Snyder County | 02 Mar 1855 | Union County | In honor of Simon Snyder (1759-1819), third Governor of Pennsylvania | Middleburg | |
| Somerset County | 17 Apr 1795 | Bedford County | The historic English county of Somerset | Somerset | |
| Sullivan County | 15 Mar 1847 | Lycoming County | In honor of John Sullivan (1740-1795); an Irish-American major general in the Continental Army in the American Revolutionary War, a delegate in the Continental Congress, Governor of New Hampshire, and a United States federal judge | Laporte | |
| Susquehanna County | 21 Feb 1810 | Luzerne County | Susquehanna River, itself named after an Algonquin word for "muddy current" | Montrose | |
| Tioga County | 26 Mar 1804 | Lycoming County | Tioga River, itself named for the Lenape word for "forks of the stream" | Wellsboro | |
| Union County | 22 Mar 1813 | Northumberland County | The federal Union of the States | Lewisburg | |
| Venango County | 12 Mar 1800 | Allegheny and Lycoming Counties | A corruption of the Lenape word "onenge", meaning "otter" | Franklin | |
| Warren County | 12 Mar 1800 | Allegheny and Lycoming Counties | In honor of Joseph Warren (1741-1775), a physician and a major general in the American Revolutionary War | Warren | |
| Washington County | 28 Mar 1781 | Westmoreland County | In honor of George Washington (1732-1799); commanding general of the Continental Army during the American Revolutionary War, a Founding Father, and first U.S. President (1789-1797) | Washington | |
| Wayne County | 21 Mar 1798 | Northampton County | In honor of Anthony "Mad Anthony" Wayne (1745-1796), a brigadier general during the American Revolutionary War | Honesdale | |
| Westmoreland County | 26 Feb 1773 | Bedford County | The historic English county of Westmorland | Greensburg | |
| Wyoming County | 04 Apr 1842 | Luzerne County | Delaware word meaning "at the big river flat" | Tunkhannock | |
| York County | 19 Aug 1749 | Lancaster County | The English city of York | York | |
