Key Biscayne, FL has a population of 12,935 and is the 3,222nd largest city in the United States. The population density is 10,371 per sq mi which is 3432% higher than the Florida average and 11347% higher than the national average. The median age in Key Biscayne is 44 which is approximately 6% higher than the Florida average of 42. In Key Biscayne, 68% of the population over 15 years of age are married, 27% speak English and 64% speak Spanish. 26% of Key Biscayne residents were born in Florida, 20% were born out of state, 5% were born outside of the United States and 49% were foreign born.