Fleming Island is an unincorporated community and census-designated place located in the U.S. state of Florida. It is within Clay County approximately 21 miles in southwest downtown Jacksonville. It has a total area of 23.1 square miles at 30°5′35″N 81°43′9″W. As of the 2010 census data, it had a total population of 27,126. The population density was 1,714/sq mi. Public education in Fleming Island is served by the Clay County School District. It has Zip code 32003.