Stock Island is an island in the lower Florida Keys immediately east of Key West. Immediately northwest is Key Haven, from which it is connected by causeway with US 1. The part north of U.S. 1 is part of the City of Key West, while the southern part is a census-designated place (CDP) in Monroe County, Florida, United States. The population of the CDP was 4,410 at the 2000 census. Stock Island was supposedly named for herds of livestock formerly kept there. Alternatively, some local historians suggest that it may be named for an early settler.