Fort Matanzas National Monument 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 (0.4 km2) 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.

The Matanzas River is a body of water in St. Johns and Flagler counties in the U.S. state of Florida. It is a narrow saltwater bar-bounded estuary sheltered from the Atlantic Ocean by Anastasia Island. The river is part of the Atlantic Intracoastal Waterway.