The R'lyeh Text is a fictional pre-human scripture introduced into the Cthulhu Mythos by August Derleth. It is described as an ancient record containing rites connected to Cthulhu and architectural secrets of the sunken city of R'lyeh.
Absent from Lovecraft's own writings, the R'lyeh Text was established through Derleth's expansions and has since been referenced by various subsequent Mythos authors.