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.