Delaware County, colloquially referred to as Delco, is a county in the Commonwealth of Pennsylvania. It is part of the Delaware Valley and borders Philadelphia. With a population of 576,830 at 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. Its county seat is Media. Until 1850, Chester was the county seat of Delaware County and, before that, of Chester County.