R'lyeh is a fictional lost city that first appeared in the H. P. Lovecraft short story "The Call of Cthulhu", first published in Weird Tales in June 1928. According to Lovecraft's short story, R'lyeh is a sunken city in the South Pacific and the prison of the entity called Cthulhu>. R'lyeh is characterized by bizarre architecture likened to non-Euclidean geometry.

Norwegian sailor Gustaf Johansen, the narrator of one of the tales in the short story, describes the accidental discovery of the city: "a coast-line of mingled mud, ooze, and weedy Cyclopean masonry which can be nothing less than the tangible substance of earth's supreme terror—the nightmare corpse-city of R'lyeh...loathsomely redolent of spheres and dimensions apart from ours". The short story also asserts the premise that while currently trapped in R'lyeh, Cthulhu will eventually return, with worshipers often repeating the phrase Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn: "In his house at R'lyeh, dead Cthulhu waits dreaming".