R'lyeh first appeared in the writings of H. P. Lovecraft. R'lyeh is also referred to in Lovecraft's "The Mound" as Relex. R'lyeh is a sunken city located deep under the Pacific Ocean and is where the godlike being Cthulhu is buried. R'lyeh's architecture is characterized by its non-Euclidean geometry. Lovecraft's "pseudo-geometry" supposes that certain shapes can extend into other dimensions; thus, appearing "non-Euclidean" from a human perspective. According to Robert Weinberg, this is impossible. Since the three-dimensional world is a closed system, no structure could be built so as to overlap into another dimension. (Robert Weinberg, "H. P. Lovecraft and Pseudomathematics", Discovering H. P. Lovecraft, pp. 88-91.) Because of the portrayal of the picture of the gorgon in "Medusa's Coil" some suppose that R'lyeh has an attribute that allows those inside to breathe underwater. Others discredit this, saying that it was merely the gorgon herself that could achieve this, or otherwise that it was merely a painting and not a reflection of actual events.
[T]he nightmare corpse-city of R'lyeh ... was built in measureless aeons behind history by the vast, loathsome shapes that seeped down from the dark stars. There lay great Cthulhu and his hordes, hidden in green slimy vaults . . .
—H. P. Lovecraft, "The Call of Cthulhu"
When R'lyeh rises in Lovecraft's short story "The Call of Cthulhu" (1928), the only portion of the city that emerges is a single "hideous monolith-crowned citadel" in which Cthulhu is entombed. The human onlookers are awed by the sheer immensity of the city and by the frightening suggestiveness of the gargantuan statues and bas-reliefs.
The city is a panorama of "vast angles and stone surfaces ... too great to belong to anything right and proper for this earth, and impious with horrible images and disturbing hieroglyphs." The geometry of R’lyeh is "abnormal, non-Euclidean, and loathsomely redolent of spheres and dimensions apart from ours."
In Lovecraft's fiction, R'lyeh is sometimes referred to in the ritualistic phrase "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn", which roughly translates to "In his house at R'lyeh, dead Cthulhu waits dreaming".<ref>Pearsall, "R'lyeh", The Lovecraft Lexicon, p. 345.</ref>
Lovecraft said that R'lyeh is located at 47°9′S, 126°43′W in the southern Pacific Ocean. (Lovecraft, "The Call of Cthulhu", The Dunwich Horror and Others, p. 150 - August Derleth), however, placed R'lyeh at 49°51′S, 128°34′Win his own writings. (Derleth, "The Black Island", Quest for Cthulhu, p. 426.) Both locations are close to the Pacific pole of inaccessibility, the point in the ocean farthest from any land. Derleth's coordinates place the city approximately 5100 nautical miles (5900 statute miles or 9500 kilometers), or about ten days journey for a fast ship, from Pohnpei (Ponape), an actual island of the area. Ponape also plays a part in the Cthulhu Mythos as the place where the "Ponape Scripture", a text describing Cthulhu, was found.
In summer 1997, the U. S. National Oceanic and Atmospheric Administration’s autonomous hydrophone array in the Equatorial Pacific Ocean repeatedly recorded a peculiar sound of a nature suggesting its biological origin. Yet, the sound’s amplitude was too large to be produced by any known animal species, and its source remains a mystery. According to NOAA, the readings yield a general location of the sound’s source “near 50° S 100° W”. The sound was given the name “Bloop”.
Charles Stross's novella 'A Colder War' implicitly locates R'lyeh in the Baltic Sea: it describes Cthulhu as being "scraped from a nest in the drowned wreckage of a city on the Baltic floor" . This is presumably because a Baltic location was more convenient for Stross's plot.
- The 1994 Doctor Who New Adventures novel All-Consuming Fire by Andy Lane features many elements inspired by the Cthulhu Mythos. Much of the action takes place on the planet "Ry'leh", which is clearly an homage, reference, or corruption of R'lyeh.
- It's mentioned in the 2nd Edition of the role-playing game Dungeons & Dragons rulebook Deities & Demigods, that any mortal seeing R'lyeh instantly becomes insane because of its unearthly construction.
- Twice in Worms of the Earth, a Bran Mak Morn tale by Robert E. Howard mention is made of the "black gods of R'lyeh". Lovecraft was a friend and correspondent of Howard and their works are littered with references to each other's creations.
- In Brian Lumley's Titus Crow novels, three psychics working for the Wilmarth Foundation attempt to locate R'lyeh psionically and are driven mad when they make contact with Cthulhu.
- R'lyeh appears as a playable nation in the turn-based strategy game Dominions II.
- In Shadow Hearts: Covenant, one of the forbidden documents is called the R'lyeh Text and explains how to summon an alien being akin to a god. However, this is a continuity error, since in Shadow Hearts this document was called the Codex of Lurie.
- The Cradle of Filth song "English Fire" from their Nymphetamine album refers to R'lyeh as one of four prophetic cities of olden times.
- Strange Tales (2) #19 features Doctor Strange battling a creature said to have come forth from the pits of R'lyeh.
- The Vision Bleak song "Kutulu!" * Carpathia - A Dramatic Poem is also a tribute to Lovecraft's story. See 
- One of Randall Flagg's pseudonyms, in Stephen King's universe. A variation of the name is mentioned in Crouch End.
- Jack Kerouac witnesses the rising of R'lyeh from the sea and goes on the road with Neal Cassady and William S. Burroughs to save the world from Cthulhu, in Nick Mamatas's novel Move Under Ground (2004).
- In Roger Zelazny's short story, "24 Views of Mt. Fuji, by Hokusai", the main character has a dream in which she encounters a temple inhabited by priests of "Old Gods", whose intent is to raise the city of R'lyeh from the depths. She also finds a cat, which she names R'lyeh.
- In Charles Stross's novella 'A Colder War', R'lyeh is found by Nazi scientists during World War II; after the war, Soviet scientists recover the not-dead-but-dreaming Cthulhu to use as the ultimate weapon of mass destruction, under the name Project Koschei, as a deterrent against the west - a rather extreme form of mutually assured destruction.
- Progressive metal band Adagio has a song called "R'Lyeh the Dead" on their 2006 album Dominate.
- The computer game X-COM: Terror from the Deep features, as its climactic final mission, an assault on the alien city of T'leth. The description of the city, shown in the screenshot to the right, has several commonalities with R'lyeh, including a powerful alien who resides in the city, neither alive nor dead, but "sleeping".
- Uncanny X-Men #148-150 features Magneto living on what appears to be the island of R'lyeh.
- The Gamecube game Eternal Darkness features a sprawling underground city apparently inspired by R'lyeh.
- The Black Phoenix Alchemy Lab has a perfume called "R'lyeh" in its "A Picnic in Arkham" collection.