Sebastian is a city in Indian River County, in the Treasure Coast region of Florida, United States. The population was 16,181 at the 2000 census. In 2004, the population recorded by the U.S. Census Bureau was 18,671. As of 2010, the population recorded by the U.S. Census Bureau is 21,929. Sebastian is a principal city of the Sebastian–Vero Beach Metropolitan Statistical Area, which includes all of Indian River County.