Fernandina Beach is a city in northeastern Florida and the county seat of Nassau County, Florida, United States. It is the northernmost city on Florida's Atlantic coast, situated on Amelia Island, and is one of the principal municipalities comprising Greater Jacksonville. The area was first inhabited by the Timucuan Indian people. Known as the "Isle of 8 Flags", Amelia Island has had the flags of the following nations flown over it: France, Spain, Great Britain, Spain (again), the Republic of East Florida (1812), the Republic of the Floridas (1817), Mexico, the Confederate States of America, and the United States.

Nassau County is the northeasternmost county of the U.S. state of Florida. According to the July 2022 United States Census analysis, the county's population was 97,899.