Fort Alcatraz was a United States Army coastal fortification on Alcatraz Island near the mouth of San Francisco Bay in California, part of the Third System of fixed fortifications, although very different from most other Third System works. Initially completed in 1859, it was also used for mustering and training recruits and new units for the Civil War from 1861 and began secondary use as a long-term military prison in 1868.

Major improvements, needed because of changes in artillery, began in the 1870s, but were underfunded and never fully completed, as the post came to be seen less as defense and more as a jail. The site was not used for any major works of the Endicott Board’s plan for the defense of the Bay.

A large concrete prison block was erected between 1910 and 1912. In 1933–1934, this was modernized and became the Alcatraz Federal Penitentiary.