Delaware County, colloquially referred to as Delco, is in the Commonwealth of Pennsylvania. With a population of 576,830 as of the 2020 census, it is the fifth-most populous county in Pennsylvania and the third-smallest in area. The county was created on September 26, 1789, from part of Chester County and named for the Delaware River. The county is part of the Southeast region of the commonwealth.
Delaware County borders Philadelphia, the nation's sixth-most populous city, to its northeast. It also is adjacent to the city-county of Philadelphia County and is included in the Philadelphia–Camden–Wilmington, PA–NJ–DE–MD metropolitan statistical area. Its county seat is Media, which is located near the center of the county.