Cthulhu is a fictional giant 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, Cthulu, Cighulu, Cathulu, Kutulu, Q'thulu, Ktulu, Kthulhut, Kulhu, Thu Thu,Harms, "Cthulhu", "PanChulhu", The Encyclopedia Cthulhiana, p. 64. 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" ( ?).Lovecraft said that "the first syllable Khlûl-hloo is pronounced gutturally and very thickly. The u is about like that in full; and the first syllable is not unlike klul in sound, hence the h represents the guttural thickness." H. P. Lovecraft, Selected Letters V, pp. 10 - 11. S. T. Joshi points out, however, that Lovecraft gave several differing pronunciations on different occasions.S. T. Joshi, note 9 to "The Call of Cthulhu, The Call of Cthulhu and Other Weird Stories According to Lovecraft, this is merely the closest that the human vocal apparatus can come to reproducing the syllables of an alien language."Cthul-Who?: How Do You Pronounce Cthulhu'?", Crypt of Cthulhu #9 Long after Lovecraft's death, the pronunciation () 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."Cthulhu Elsewhere in Lovecraft", Crypt of Cthulhu #9. 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.

The Cthulhu Mythos is a shared universe created in the 1920s by American horror writer H. P. Lovecraft. The term Lovecraft Mythos is preferred by some — most notably the Lovecraft scholar S.T. Joshi.Joshi, "The Lovecraft Mythos", H. P. Lovecraft, p. 31ff.

The term was coined by Lovecraft's associate August Derleth, and named after Cthulhu, a powerful fictional entity in Lovecraft's stories. Not a series per se, stories, novels and other works in the Cthulhu Mythos feature elements, characters, settings, and themes found in works by Lovecraft writers. Together, these works form the Category: wikt - :mythos|mythos that authors writing in the Lovecraftian milieu have used -- and continue to use -- in their ongoing expansion of the fictional universe, sometimes in ways far removed from Lovecraft's original conception.Harms, "A Brief History of the Cthulhu Mythos", The Encyclopedia Cthulhiana, pp. viii?ix.