Callahan is a town in Nassau County, Florida, United States, adjacent to Duval County. The population was 1,123 at the 2010 census. It is one location of the Northeast Florida Fair. The Battle of Alligator Bridge took place 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. The median household income was $25,234.