Nassau County is a county located in the U.S. state of Florida. Its county seat is Fernandina Beach. Nassau County is part of the Greater Jacksonville Metropolitan area. Nassau County was created in 1824 and was named for the Duchy of Nassau in Germany. Nassau County has a total area of approximately 726 square miles of which 90% is land and 10% water. As of the 2000 census, there were 57,663 people, 21,980 households, and 16,528 families residing in Nassau County, FL.