The Deep Ones are creatures in the Cthulhu Mythos of H. P. Lovecraft. The beings first appeared in Lovecraft's novella The Shadow over Innsmouth (1931), but were already hinted at in the early short story "Dagon". The Deep Ones are a race of intelligent ocean-dwelling creatures, approximately human-shaped but with a fishy appearance. The males would regularly rape human women along the coast, creating societies of hybrids.

Numerous Mythos elements are associated with the Deep Ones, including the legendary town of Innsmouth, the undersea city of Y'ha-nthlei, the Esoteric Order of Dagon, and the beings known as Father Dagon and Mother Hydra. After their debut in Lovecraft's tale, the sea-dwelling creatures resurfaced in the works of other authors, especially August Derleth.

The Deep Ones are an ancient species of amphibious sea-dwelling humanoids, whose preferred habitat is the deep ocean. A description is offered by the narrator of The Shadow Over Innsmouth:

I think their predominant color was a greyish-green, though they had white bellies. They were mostly shiny and slippery, but the ridges of their backs were scaly. Their forms vaguely suggested the anthropoid, while their heads were the heads of fish, with prodigious bulging eyes that never closed. At the sides of their necks were palpitating gills, and their long paws were webbed. They hopped irregularly, sometimes on two legs and sometimes on four. I was somehow glad that they had no more than four limbs. Their croaking, baying voices, clearly used for articulate speech, held all the dark shades of expression which their staring faces lacked ... They were the blasphemous fish-frogs of the nameless design—living and horrible.

A very similar description is provided in the much-earlier story Dagon:

I think that these things were supposed to depict men—at least, a certain sort of men; though the creatures were shewn disporting like fishes in the waters of some marine grotto, or paying homage at some monolithic shrine which appeared to be under the waves as well. Of their faces and forms I dare not speak in detail; for the mere remembrance makes me grow faint. Grotesque beyond the imagination of a Poe or a Bulwer, they were damnably human in general outline despite webbed hands and feet, shockingly wide and flabby lips, glassy, bulging eyes, and other features less pleasant to recall. Curiously enough, they seemed to have been chiselled badly out of proportion with their scenic background; for one of the creatures was shewn in the act of killing a whale represented as but little larger than himself.

A significant difference between the two narratives is that the single creature observed by the narrator is of a vast size, whereas the beings seen off Innsmouth are roughly human in scale. Despite being primarily marine creatures, Deep Ones can survive on land for extended periods of time. They possess biological immortality, and never die except by accident or violence. They worship twin deities, the cult of whom they have introduced among the human population of Innsmouth, who know them as "Father Dagon and Mother Hydra". However, the elderly derelict (and Order of Dagon initiate) Zadok Allen invokes Cthulhu in a moment of strong emotion, and Robert M. Price has suggested that "Dagon" may have merely been "the closest biblical analogy to the real object of worship of the deep ones" The Deep Ones are or were opposed by mysterious beings known as the Old Ones, who have left behind magical artifacts that can keep them in check. This detail is one of the vestigial hints that August Derleth developed into the mostly unnamed Elder Gods.

The Esoteric Order of Dagon was the primary religion in Innsmouth after Marsh returned from the South Seas with the dark religion circa 1838. It quickly took root due to its promises of expensive gold artifacts and fish. Fish were a valuable commodity in Innsmouth, a fishing town.