Gormenghast is a fantasy series by British author Mervyn Peake, about the inhabitants of Castle Gormenghast, a sprawling, decaying, Gothic structure. Originally conceived as a single on-going novel, the series was ended by Peake's death and comprises three novels: Titus Groan (1946), Gormenghast (1950) and Titus Alone (1959); and a novella, Boy in Darkness (1956), whose canonical status is debated. Peake was writing a fourth novel, Titus Awakes, at the time of his death in 1968. The book was completed by Peake's widow Maeve Gilmore in the early 1970s. After it was discovered by their family, it was published in 2009.

Although the first two instalments do not contain any overtly fantastical elements, Gormenghast is almost unanimously categorised as fantasy because of the atmosphere and pseudo-medieval setting. The series has received widespread acclaim from the speculative fiction community and mainstream literary critics.

The series has been included in Fantasy: The 100 Best Books, Modern Fantasy: The 100 Best Novels and 100 Must Read Fantasy Novels as one of the greatest fantasy works of the twentieth century. Literary critic Harold Bloom has praised the series as the best fantasy novels of the 20th century and one of the greatest sequences in modern world literature. Gormenghast is often credited as the first fantasy of manners novel. The books have been translated into over twenty languages.