Alcatraz Water Tower, Water tower at Alcatraz Island, San Francisco, United States.

The Alcatraz Water Tower is a cylindrical tank on a steel lattice tower of six cross-braced legs set in concrete foundations, on the northwestern side of Alcatraz Island in San Francisco Bay. The tank has a slightly domed top and a shallow conical bottom, and its pale metal skin is streaked with rust. A small white shed with a flat roof and dark door stands at ground level within the open framework.

The tower was built between 1940 and 1941 by the Federal Bureau of Prisons to supply water to the penitentiary. After the prison closed in 1963, it was kept as part of the protected historic site.

During the Native American occupation from 1969 to 1971, protesters painted messages on the tower that are still visible today. These painted words remind visitors that the island was also a place of protest, not only a prison.

The tower cannot be entered but is easy to see and photograph from the island's walking paths. The terrain on the island is hilly, so comfortable footwear makes the visit more manageable.

The tower's tank holds 250,000 U.S. gallons (950,000 liters) of fresh water and stands 94 feet (29 meters) tall, making it the tallest building on Alcatraz. It was built in 1940–41 by the Federal Bureau of Prisons after a federal grant established a freshwater supply on the island. The tank rests on six cross-braced steel legs set in concrete foundations.