Stock Island is a census-designated place (CDP) & Unincorporated community on an island of the same name in Monroe County, Florida, United States. The population of the CDP was 4,410 at the 2000 census. It is located on the portion of the island south of US 1. It is supposedly named for the herds of livestock formerly kept there. Alternatively, some local historians suggest that it may be named for an early settler.