Sebastian is a city in Indian River County, Florida, United States. The population was 16,181 at the 2000 census. As of 2004, the population recorded by the U.S. Census Bureau is 18,671 United States Census Bureau, States the city of Sebastian has a total area of 35 Sq Kilometers, but to the people of Sebastian its miles away from normal, That’s what happens when you take a couple of schools, roughly 16 thousand people and a climate any Chamber of Commerce would be proud to say was theirs.