Cthulhu is a giant fictional creature, one of the Great Old Ones in H. P. Lovecraft's Cthulhu Mythos. It is often cited for the extreme descriptions given of its appearance, size, and the abject terror that it invokes. Cthulhu is often referred to in science fiction and fantasy circles as a tongue-in-cheek shorthand for extreme horror or evil.

Cthulhu has also been spelled as Tulu, Clulu, Clooloo, Cighulu, Cathulu, Kutulu, Q’thulu, Ktulu, Kthulhut, Kulhu, Thu Thu, and in many other ways. It is often preceded by the epithet Great, Dead, or Dread.

Lovecraft transcribed the pronunciation of Cthulhu as "Khlûl'hloo". S. T. Joshi points out, however, that Lovecraft gave several differing pronunciations on different occasions. According to Lovecraft, this is merely the closest that the human vocal apparatus can come to reproducing the syllables of an alien language. Long after Lovecraft's death, the pronunciation kə-THOO-loo became common, and the game Call of Cthulhu endorsed it.

Cthulhu first appeared in the short story "The Call of Cthulhu" (1928)—though it makes minor appearances in a few other Lovecraft works. August Derleth, a correspondent of Lovecraft's, used the creature's name to identify the system of lore employed by Lovecraft and his literary successors, the Cthulhu Mythos.