Forte de São Mateus do Cabo Frio, Portuguese colonial fortress in Cabo Frio, Brazil.

Forte de São Mateus do Cabo Frio sits on an elevated position overlooking Cabo Frio bay and the Atlantic coast. The structure features thick stone walls, angled bastions, and interior chambers that reveal its purpose as a military installation.

The fort was designed in 1617 by engineer Francisco de Frias da Mesquita, replacing the vulnerable Forte de Santo Inácio, and was already operating by 1620. In 1648 its artillery and garrison were withdrawn for the reconquest of Angola, and Estêvão Gomes rearmed it in 1650. From 1899 the building served as a lazaretto, and by the late 1930s it lay in ruins.

The fortress demonstrates how the Portuguese asserted control over the Brazilian coast and the military architectural styles they introduced. Visitors can see how the cannon placements and defensive layouts reflected this strategic approach.

The fort stands on a rocky islet at the northeastern end of Praia do Forte, reached from the beach rather than from the town centre. The ground is uneven rock and masonry, and the five interior compartments include a cistern at a lower level. The site is exposed to sun and sea wind, so early morning or late afternoon visits are cooler.

Five large-caliber iron cannons ordered during Emperor Pedro II's rearmament of Brazilian fortresses before 1889 still stand in the fort. The building later served as a lazaretto from 1899, housing terminal patients of epidemics that struck Cabo Frio.