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.

Fort Matanzas was built by the Spanish in 1742 to guard Matanzas Inlet, the southern mouth of the Matanzas River, which could be used as a rear entrance to the city of St. Augustine. Such an approach avoided St. Augustine's primary defense system, centered at Castillo de San Marcos. In 1740, Gov. James Oglethorpe of Georgia used the inlet to blockade St. Augustine and launch a thirty-nine-day siege. St. Augustine endured the siege, but the episode convinced the Spanish that protecting the inlet was necessary to the security of the town. Under Gov. Manuel de Montiano's orders, construction of the fort began that year and was completed in 1742. Engineer Pedro Ruiz de Olano, who had worked on additions to the Castillo de San Marcos, designed the fortified observation tower. Convicts, slaves, and troops from Cuba were used as labor to erect the structure, which was sited on present-day Rattlesnake Island and had a commanding position over Matanzas Inlet.

The fort, known to the Spanish as Torre de Matanzas (Matanzas Tower), is a masonry structure made of coquina, a common shellstone building material in the area. The marshy terrain was stabilized by a foundation of pine pilings to accommodate a building 50 feet (15 m) long on each side with a 30-foot (9.1 m) high tower. The standard garrison of the fort was one officer in charge, four infantrymen, and two gunners, though more troops could be stationed if necessary. All soldiers at Fort Matanzas served on rotation from their regular duty in St. Augustine. Five cannon were placed at the fort—four six-pounders and one eighteen-pounder. All guns could reach the inlet, which at the time was less than half a mile away.

In 1742, as the fort was nearing completion, the British under Oglethorpe approached the inlet with twelve ships. Cannon fire drove off the scouting boats, and the warships left without engaging the fort. This brief encounter was the only time Fort Matanzas fired on an enemy. Spain lost control of Florida with the 1763 Treaty of Paris, and regained control with the 1783 Treaty of Paris. With the Spanish Empire falling apart, Spain spent little effort maintaining the fort after this time. When the United States took control of Florida in 1821, the fort had deteriorated to the point where soldiers could not live inside. The United States never used the fort and it became a ruin.

Fort Matanzas was named for the inlet, which acquired its name after the executions, or matanzas (Spanish: slaughters), on its north shore, of Jean Ribault and his band of Huguenot Frenchmen, the last of the Fort Caroline colonists, by the Spanish in 1565