Stock Island is a census-designated place and unincorporated community on the island of the same name in Monroe County, Florida, United States. The population of the CDP was 4,722 at the 2020 census, up from 3,919 in 2010. 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.