About 20ft (6m) of lead drain pipework was stolen from the Black Barge Beach side of Portland Castle in Dorset.
It is unclear when the offenders struck as the theft was only discovered on Friday evening, police said.
English Heritage, which manages the attraction, said the seaside fort was built in the early 1540s to protect against French and Spanish invasion.
A castle dating back to King Henry VIII's reign has been targeted by thieves.