Fort Matanzas National Monument is the site where the Spanish built a fort. It was designated a United States National Monument on October 15, 1924. The monument consists of a 1740 Spanish fort called Fort Matanzas, and about 100 acres of salt marsh and barrier islands along the Matanzas River on the northern Atlantic coast of Florida. It is operated by the National Park Service in conjunction with the Castillo de San Marcos National Monument in the city of St. Augustine.