Nassau County was created in 1824  - The county is located just above Duval County, Florida

Nassau County is the northeasternmost county of the U.S. state of Florida. According to the United States Census, the county's population was 88,625 in 2019.

The county seat and the largest incorporated city is Fernandina Beach. Nassau County is part of the Jacksonville metropolitan area, which was home to 1,534,701 people in 2018. The county is situated in Northeast Florida with a land area of 726 square miles (1,880 km²). Population growth in the county has increased by over 28,000 residents since the year 2000 as a result of Nassau's proximity to downtown Jacksonville, new housing developments, agricultural production, tourism locations, and a diversifying tax base with new industrial and commercial companies moving to the county. Nassau County is also a popular choice of residence for military personnel stationed on bases in nearby Duval County (Naval Air Station Jacksonville, Naval Station Mayport) and Camden County (Naval Submarine Base Kings Bay).

The Battle of Alligator Bridge took place in Nassau County around Callahan on June 30, 1778, and was the only major engagement in an unsuccessful campaign to conquer British East Florida during the American Revolutionary War.