The Castle of Otranto is a 1764 novel by Horace Walpole, first published under a pseudonym. It is generally regarded as the first ever gothic novel, initiating a literary genre that would become extremely popular in the later 18th century and early 19th century.

The novel tells the story of Manfred, lord of the castle, and his family. Manfred's sickly son Conrad is set to be wed to princess Isabella, but Conrad is crushed to death by an enormous helmet before the ceremony ever begins. This inexplicable event links back to an ancient prophecy: "That the castle and lordship of Otranto should pass from the present family, whenever the real owner should be grown too large to inhabit it." Manfred, terrified that Conrad's death signals the beginning of the end for his family line, resolves to avert destruction by marrying Isabella himself while divorcing his current wife Hippolita. In the ensuing chaos, knights gallant and unexpected heirs abound.