English Heritage said divers stole the torpedo tube hatch of the Holland 5, which sank six miles off Eastbourne in East Sussex in 1912.
The theft was discovered during a licensed dive by the Nautical Archaeology Society (NAS) in June and confirmed during a dive last month.
The NAS described the wreck as a “remarkable piece of naval heritage”.
Sussex Police and English Heritage have appealed for help to catch the perpetrators, who may have struck up to two years ago.
Experts said a group of people would have been behind the theft but that the hatch carried very little monetary value.