New Castle is one of the 3 counties in the state of Delaware. It was created in 1664, making it an original county of the state. As of the 2010 census, the population was 538,479, up from 500,265 in 2000. The county seat and largest city is Wilmington. It has an area of 493.51 square miles (1,278 km2), of which 426.27 square miles (1,104 km2) is dry land. It is the smallest of the three counties by size, but has more than 60% of the state's population.
New Castle is bordered by Kent County to the south, Kent County, Maryland to the southwest, Cecil County, Maryland to the west, Chester County, Pennsylvania to the northwest, Delaware County, Pennsylvania to the north and the Delaware River to the east, although there is a bridge connection to Salem County, New Jersey to the east, and technically there is a river border to Gloucester County, New Jersey to the northeast.