Jacksonville, city, seat (1822) of Duval county, northeastern Florida, U.S., the centre of Florida’s “First Coast” region. It lies along the St. Johns River near its mouth on the Atlantic Ocean, about 25 miles (40 km) south of the Georgia border. Jacksonville consolidated (1968) with most of Duval county and thereby became one of the nation’s largest cities in area (841 square miles [2,178 square km]).