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.The designated county seat is Fernandina Beach. Nassau County is a constituent of the Jacksonville metropolitan area, which accommodates around 1.68 million inhabitants as of the year 2022. The county is situated in Northeast Florida with a land area of 726 square miles.