"Old One" is an ambiguous term in the Cthulhu Mythos, and may refer to:
- Great Old Ones, a pantheon of dead alien deities buried inside the Earth, including, Cthulhu. (The Call of Cthulhu).
- Outer Gods, a pantheon of deities who break inside the universe through Yog-Sothoth (The Dunwich Horror).
- Entities who can defeat the Deep Ones using a swastika symbol (The Shadow Over Innsmouth).
- Inhabitants of K'n-yan (The Mound).
- Elder Things, a species of alien lifeforms (At the Mountains of Madness).
- The inhabitants of the City of the Winged One, whose last representative slaughtered the Black Corsairs and Bêlit, and was killed by Conan (Queen of the Black Coast).
Other groups and species within the Marvel Multiverse version of the Mythos are known as Old Ones:
- The Unforgiven Dead, a mysterious race that ruled Atlantis and sacrifice Atlanteans to the Old Ones.
- The first of the Aqueos.