Delaware County, colloquially referred to as Delco, is a county located in the U.S. state of Pennsylvania. With a 2010 census population of 558,979, it is the fifth most populous county in Pennsylvania, and the third most compact. Its county seat is Media. The county was created on September 26, 1789, from part of Chester County and named for the Delaware River. Chester City, prior to 1851, was the county seat of both Delaware County and, before that, of Chester County.