Philadelphia County is the most populous of the 67 counties of Pennsylvania and the 24th-most populous county in the nation. As of the 2020 census, the county had a population of 1,603,797. It is coextensive with Philadelphia, the nation's sixth-largest city. The county is part of the Southeast Pennsylvania region of the state.

Philadelphia County is one of the three original counties, along with Chester and Bucks counties, founded by William Penn in November 1682. Since 1854, the county has been coextensive with the City of Philadelphia, which is also its county seat. Philadelphia County is the core county in the Philadelphia-Camden-Wilmington Combined Statistical Area (PA-NJ-DE-MD, located along the lower Delaware and Schuylkill Rivers, within the Northeast megalopolis.)

Philadelphia County is the Delaware Valley's economic and cultural anchor and the ninth-largest combined statistical area in the nation with an estimated population of 7,381,180 as of 2022.^{[circular reference]}

Native American tribes of Lenape were the first known occupants in the area that became Philadelphia County. The first European settlers were Swedes and Finns who arrived in 1638. The Netherlands seized the area in 1655, but lost control to England in 1674. William Penn received his charter for Pennsylvania from Charles II of England in 1681, and in November 1682 he divided Pennsylvania into three counties. During the same year, Philadelphia was planned and was made the county seat and the capital of the Province of Pennsylvania.

Penn wanted Philadelphia, meaning "love brotherly", to be a place where religious tolerance and the freedom to worship were ensured. Philadelphia's name is shared with an ancient city in Asia Minor mentioned by the Bible's Book of Revelation. It was William Penn's desire, as a Quaker, that his "Holy Experiment" would be found blameless at the Last Judgment.

When established, Philadelphia County consisted mainly of the area from the Delaware River west between the Schuylkill River to the south and the border with Bucks County to the north; the western boundary was undefined. Two counties would be formed out of Philadelphia County, Berks County which was formed in 1752 (from parts of Chester, Lancaster, and Philadelphia counties), and Montgomery County established in 1784. From these separations, as well as other border changes, was created the present-day boundaries of the county.

Philadelphia, as planned by Penn, comprised only that portion of the present-day city situated between South and Vine Streets and the Delaware and Schuylkill Rivers. Other settlements were made beyond the boundaries of the city, and in the course of time they became incorporated separately and had separate governments.

Several of these settlements were situated immediately contiguous to the City of Philadelphia, such as Southwark and Moyamensing in the south, the Northern Liberties District, Kensington, Spring Garden and Penn District to the north, and West Philadelphia and Blockley to the west — which combined with the City of Philadelphia formed practically one continuously urban area, the whole group being known abroad simply as Philadelphia.