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 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. Augustine Inlet.
Part of the island is within St. Augustine city limits, while other communities on the island include St. Augustine Beach, Coquina Gables, Butler Beach, Crescent Beach, and Treasure Beach.
Fort Matanzas National Monument, a Spanish colonial-era fort built in 1740–1742, is located at the southern end of the island on Rattlesnake Island in the Intracoastal waterway within the park boundaries; it was designed to protect St. Augustine from attack via the Matanzas River.