Delaware County is a county located in the U.S. state of Pennsylvania. As of 2000, the population was 550,864, making it Pennsylvania's fifth most populous county, behind Philadelphia, Allegheny, Montgomery, and Bucks counties.
Delaware County was created on September 26, 1789, from part of Chester County and named for the Delaware River. Its county seat, since 1851, is Media. Chester City, prior to 1851, was the county seat of both Delaware County and, before that, of Chester County.
Delaware County consists of suburbs of the adjacent city of Philadelphia. Socioeconomically, Delaware County ranges from economically depressed and working class communities such as the City of Chester and the Borough of Darby to the prosperous suburban neighborhoods of Radnor Township within the wealthy Main Line to the north.
2,600 acres (11 km2) of the county are occupied by the Ridley Creek State Park.
The county, immediately adjacent to and west of Philadelphia, is part of the Delaware Valley area. Delaware County is only one of two counties covered in its entirety by area code 610 (the other one being Chester County).