The Castle Conundrum is a Hardy Boys novel. It was first published in 2001.

The Hardys head to France to help in the construction of a center for children, thanks to Teen Village International, a worldwide, non-profit organization. But the Hardys soon realize that the castle next door is supposedly haunted. The Hardys soon realize that someone is after the treasure hidden inside the castle, and now, the Hardys must find out who, before he or she gets rid of them for good.