R'lyeh (pronounced Roo-lee-ah) is a fictional city that first appeared in the writings of H. P. Lovecraft. R'lyeh is a sunken city deep under the Pacific Ocean where the godlike being Cthulhu resides. R'lyeh is characterized by an architecture based on non-Euclidean geometry.

Co-ordinates of S. Latitude 47° 9′, W. Longitude 126° 43′ have been stated by Lovecraft but never investigated. August Derleth used the co-ordinates of S. Latitude 49° 51′, W. Longitude 128° 34′ in his own writings. These coordinates place the city in the middle of one of the biggest patches of empty ocean on Earth. The latter also places it about a day's journey from Pohnpei (Ponape), an actual island of the area, which consequently plays a part in the Cthulhu Mythos as the origin of the "Ponape Scripture ", a text describing Cthulhu.

In Lovecraft's fictional Cthulhu Mythos, the most ubiquitous liturgical phrase is "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn!" -- "In his house at R'lyeh, dead Cthulhu waits [or lies] dreaming."