Philadelphia County is the most populous of Pennsylvania's 67 counties and the 24th-most populous in the United States. It is coextensive with the City of Philadelphia, which serves as its county seat and is the nation's sixth-largest city. Located in the Southeast Pennsylvania region along the lower Delaware and Schuylkill Rivers, the county forms the core of the Philadelphia-Camden-Wilmington Combined Statistical Area within the Northeast megalopolis.

William Penn founded the county in November 1682 as one of three original counties in Pennsylvania. Philadelphia was planned that same year as the capital of the Province of Pennsylvania, intended to ensure religious tolerance and freedom of worship. The county's boundaries evolved through the formation of Berks County in 1752 and Montgomery County in 1784. In 1854, a reform charter consolidated all boroughs and townships into the City of Philadelphia, effectively merging city and county governments.

The city is defined by districts such as Center City and Old City, which comprise the downtown section bounded by Vine Street and the rivers. It is known for its role in American democracy, hosting Independence Hall where the Revolution was planned. The area features firsts including the nation's first library, hospital, medical school, and stock exchange. The landscape includes the Delaware River, Schuylkill River, and varied terrain from Fort Mifflin to Chestnut Hill.

Philadelphia serves as the Delaware Valley's economic and cultural anchor, attracting visitors with its rich historical heritage. The city celebrates its status as the birthplace of American democracy and modern liberty. Its diverse population includes significant communities from Latin America, Asia, and Europe, contributing to a vibrant cultural landscape. The region offers extensive museums, historic sites, and a dynamic urban environment that reflects its colonial and revolutionary past.

As of the 2020 census, the county had a population of 1,603,797. It is classified as a First Class County due to its size and functions as the economic hub of the region. The area is accessible via major roads and highways connecting it to New Jersey and Delaware. With a median age of 34.8 years, the county maintains a dense urban character, supported by a robust infrastructure and diverse demographic makeup that continues to grow.