Key Biscayne is an island town in Miami-Dade County, Florida, United States. The population was 12,344 at the 2010 census.Key Biscayne lies south of Miami Beach and east of Miami. The village is connected to Miami via the Rickenbacker Causeway, originally built in 1947. Because of its low elevation and direct exposure to the Atlantic Ocean, it is usually among the first Miami areas to be evacuated before an oncoming hurricane.