Fortaleza de São José da Ilha das Cobras, Colonial fortress in Ilha das Cobras, Brazil.

Fortaleza de São José da Ilha das Cobras is a military fortress on an island in Rio de Janeiro's bay, featuring stone walls, defensive structures, and a chapel. The design follows classical European fortress patterns, though it was built using locally available stone and materials.

Portuguese engineers built this military fortress in 1624 to protect Rio de Janeiro's harbor from Dutch attacks during the Portuguese-Dutch conflict. The fortification was part of a larger strategy to secure the colony's stability along the South American coast.

The island's name comes from the many snakes (cobras) found there when the Portuguese arrived. The 1736 plan by engineer José da Silva Paes gave the slopes an inaccessible, castle-like look and split the defence into three forts: São José, Pau da Bandeira and Santo António. The São José terreplein held the governor's house, a chapel, the powder magazine and the guardhouse.

The fortress now serves as the Navy's Central Hospital and remains in active use today. Visitors should know that access may be limited and advance notice is often required.

In 1718 governor Antônio de Brito Freire de Menezes reported that the island's fortification held 26 guns and 716 cannonballs. French corsair René Duguay-Trouin, who took the city in 1711, wrote that he found two fortifications on the island; a French source says the battery had seven pieces, raised to 12 cannons and five mortars in French hands.