The Alcatraz Federal Penitentiary or United States Penitentiary, Alcatraz Island, also known simply as Alcatraz (English: , Spanish: [alkaˈtɾas] "the gasset") or The Rock was a maximum security federal prison on Alcatraz Island, 1.25 miles (2.01 km) off the coast of San Francisco, California, United States, the site of a fort since the 1850s; the main prison building was built in 1910–1912 as a United States Army military prison.

The United States Department of Justice acquired the United States Disciplinary Barracks, Pacific Branch, on Alcatraz on 12 October 1933. The island became adapted and used as a prison of the Federal Bureau of Prisons in August 1934 after the buildings were modernized and security increased. Given this high security and the island's location in the cold waters and strong currents of San Francisco Bay, prison operators believed Alcatraz to be escape-proof and America's strongest prison.

The three-story cellhouse included the four main cell blocks – A-block through D-block, the warden's office, visitation room, the library, and the barber shop. The prison cells typically measured 9 feet (2.7 m) by 5 feet (1.5 m) and 7 feet (2.1 m) high. The cells were primitive and lacked privacy. They were furnished with a bed, desk, washbasin, a toilet on the back wall, and few items other than a blanket. African Americans were segregated from other inmates in cell designation due to racism during the Jim Crow-era.

D-Block housed the worst inmates, and six cells at its end were designated "The Hole". Prisoners with behavioral problems were sent to these for periods of often brutal punishment. The dining hall and kitchen extended from the main building. Prisoners and staff ate three meals a day together. The Alcatraz Hospital was located above the dining hall.

Prison corridors were named after major U.S. streets, such as Broadway and Michigan Avenue, of New York and Chicago, respectively. Working at the prison was considered a privilege for inmates. Those who earned privileges were employed in the Model Industries Building and New Industries Building during the day, actively involved in providing for the military in jobs such as sewing and woodwork, and performing various maintenance and laundry chores.

After being closed in 1963 as a prison, Alcatraz has been reopened as a public museum. It is one of San Francisco's major tourist attractions, attracting some 1.5 million visitors annually. Now operated by the National Park Service's Golden Gate National Recreation Area, the former prison is being restored and maintained.