Nassau County is the northeasternmost county of the U.S. state of Florida. The United States Census Bureau estimates that the county's population was 101,501 on July 1, 2023, up from 90,352 at the 2020 census. The designated county seat is Fernandina Beach. Nassau County is a constituent of the Jacksonville, Florida Metropolitan Statistical Area, which accommodates around 1.68 million inhabitants as of the year 2022.