Deep Ones are a race of undersea-dwelling humanoids whose preferred habitat is deep in the ocean (hence their name). However, despite being primarily marine creatures, they can come to the surface and can survive on land for some time. All Deep Ones are immortal; none die except by accident or violence. They are said to serve the beings known as Father Dagon and Mother Hydra, as well as Cthulhu. They are opposed by mysterious beings known as the Old Gods, whose powerful magic can keep them in check.

Deep Ones pass through several stages of development. At the earliest stages they appear as overgrown tadpoles. One such creature was discovered in the Medusa Coil, see Deep One in a Jar