Philadelphia County is a county located in the commonwealth of Pennsylvania. At the 2010 census, the population was 1,526,006, making it the most populous county in the state, despite being the second smallest county in the state by land area. Philadelphia County is one of the three original counties, along with Chester and Bucks counties, created by William Penn in November 1682. Since 1854, the county has been coterminous with the city of Philadelphia, which also serves as its seat. It is the most populous county in the United States coterminous with a municipality.