Anastasia Island is a barrier island located off the northeast Atlantic coast of Florida in the United States. It sits east of St. Augustine, running north-south in a slightly southeastern direction to Matanzas Inlet. The island is about 14 miles (23 km) long and an average of 1 mile in width. It is separated from the mainland by the Matanzas River, part of the Intracoastal waterway. Matanzas Bay, the body of water between the island and downtown St. Augustine, opens into St.