Guantanamo Bay Naval Base (Spanish: Base Naval de la Bahía de Guantánamo), officially known as Naval Station Guantanamo Bay or NSGB, (also called GTMO, pronounced Gitmo GIT-moh as jargon by members of the U.S. military) is a United States military base located on 45 square miles (117 km2) of land and water on the shore of Guantánamo Bay at the southeastern end of Cuba. It has been leased to the United States with no end date since 1903 as a coaling station and naval base, making it the oldest overseas U.S. naval base. The lease was $2,000 in gold per year until 1934, when the payment was set to match the value of gold in dollars; in 1974, the yearly lease was set to $4,085.

Since taking power in 1959, the Cuban communist government has consistently protested against the U.S. presence on Cuban soil, arguing that the base "was imposed on Cuba by force" and is "illegal under international law." Since 2002, the naval base has contained a military prison, for alleged unlawful combatants captured in Afghanistan, Iraq, and other places during the War on Terror. Cases of alleged torture of prisoners by the U.S. military, and their denial of protection under the Geneva Conventions, have been criticized.

The 1903 lease has no fixed expiration date; as such, it can only be ended if the US Navy decided to abandon the area or both countries agreed mutually to end the lease.