Cthulhu Companion is a supplement with information on prisons, insanity, Mythos creatures, and the Necronomicon, and also contains poetry from H.P.

[1] Cthulhu Companion is a collection of essays written by Sandy Petersen, Glenn Rahman, Lynn Willis, Morgan Conrad, Alan K. Crandall, Gene Day, William Hamblin, Keith Herber, Chris Marrinan, John Sullivan, Tom Sullivan, and Richard L. Tierney, and edited by Sandy Petersen and Yurek Chodak.

[2][3] Jon Sutherland reviewed Cthulhu Companion for White Dwarf #51, giving it an overall rating of 7 out of 10, and stated that "In conclusion, this tome is really of use only to the Keepers of Arcane Knowledge and given that this does not set out to fundamentally change any of the basic rules themselves, again this will limit appeal.

"[5] In the September–October 1984 edition of Different Worlds (Issue #36), Steve Marsh was slightly disappointed about this product, commenting that some of the material was very good, but some essays were of more questionable utility.

"[6] James Maliszewski for Black Gate in 2014 said "Whereas the original 1981 rulebook depicted Call of Cthulhu investigators as reporters and academics, the Companion gave the impression of their being more like Indiana Jones: bold explorers venturing into forgotten tombs in search of treasure and knowledge.