Alcatraz Island is located in the San Francisco Bay, 2.4 km offshore from San Francisco, California, United States. Often referred to as "The Rock", the small island was developed with facilities for a lighthouse, a military fortification, a military prison (1868), and a federal prison from 1933 until 1963.

Native Americans kept well away from the island, calling it "Evil Island" and believing it to be cursed. The first Spaniard to document the island was Juan Manuel de Ayala in 1775, who charted San Francisco Bay and named one of the three islands he identified as the "La Isla de los Alcatraces," which translates as "The Island of the Pelicans" from the archaic Spanish alcatraz (in English: "pelican"). Over the years, the English version "Alcatraz" became popular and is now widely used. The Spanish put a few small buildings on the island, little else.

Because of its isolation from the outside by the cold, strong, hazardous currents of the waters of San Francisco Bay, Alcatraz was used to house Civil War prisoners as early as 1861.

Following the war in 1866, the army switched the focus of its plans for Alcatraz from coastal defense to detention, a task for which it was well suited because of its isolation. In 1867, a brick jailhouse was built, and in 1868, Alcatraz was officially designated a long-term detention facility for military prisoners.

In 1909 construction began on the huge concrete main cell block, which remains the island's dominant feature. It was completed in 1912. The building had been constructed in an excavated pit (creating a dry "moat") to enhance its defensive potential. The first floor was then incorporated as a basement to the new cell block, giving rise to the popular legend of "dungeons" below the main cell block. The Fortress was deactivated as a military prison in October 1933 and transferred to the Bureau of Prisons. Alcatraz was designed to hold prisoners who continuously caused trouble at other federal prisons. During the 29 years it was in use, the jail held some of the most notorious criminals in American history, such as Al Capone, Robert Franklin Stroud (the Birdman of Alcatraz), or George "Machine Gun" Kelly. During its 29 years of operation, the penitentiary claimed that no prisoner successfully escaped. A total of 36 prisoners made 14 escape attempts, two men trying twice; 23 were caught, six were shot and killed during their escape, two drowned, and five are listed as "missing and presumed drowned".

Today, the island's facilities are managed by the National Park Service as part of the Golden Gate National Recreation Area; it is open to tours. Visitors can reach the island by ferry ride.