Castillo de San Marcos in the context of Pedro Menéndez de Avilés

The United States has 138 protected areas known as national monuments. The president of the United States can establish a national monument by presidential proclamation, and the United States Congress can do so by legislation. The president's authority arises from the Antiquities Act of 1906, which allows the president to proclaim "historic landmarks, historic and prehistoric structures, and other objects of historic or scientific interest" as national monuments.

Concerns about protecting mostly prehistoric Native American ruins and artifacts, collectively known as antiquities, on western federal lands prompted the legislation, which allowed the president to quickly preserve public land without waiting for legislation to pass through an unconcerned Congress. The ultimate goal was to protect all historic and prehistoric sites on U.S. federal lands, and it has resulted in designation of a wide variety of ecological, cultural and historical sites.

Fort Matanzas National Monument (Spanish: Fuerte Matanzas) 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 (0.4 km) 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.