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

Philadelphia, commonly referred to as Philly, is the most populous city in the U.S. state of Pennsylvania. Philadelphia is the nation's sixth-most populous city, with a population of 1,603,797 in the 2020 census and is the urban core of the larger Delaware Valley, the nation's seventh-largest and one of the world's largest metropolitan regions consisting of 6.245 million residents in the metropolitan statistical area and 7.366 million residents in its combined statistical area. Philadelphia is known for its extensive contributions to United States history, especially the American Revolution, and served as the nation's capital until 1800. It maintains contemporary influence in business and industry, culture, sports, and music.