Fleming Island is an unincorporated community and census-designated place in Clay County, Florida, United States. It is located 21 miles southwest of downtown Jacksonville, on the western side of the St. Johns River, off US 17. As of the 2020 census the Fleming Island CDP had a population of 29,142, up from 27,126 at the 2010 census. It is part of the Jacksonville, Florida Metropolitan Statistical Area.