Chaosium Inc. (/keɪˈɒsiəm/ kay-OSS-ee-əm) is a publisher of tabletop role-playing games established by Greg Stafford in 1975. Chaosium's major product lines include RuneQuest, set in Stafford's fictional world of Glorantha; Call of Cthulhu, based on the horror fiction stories of H. P. Lovecraft; Pendragon, based on Thomas Mallory's Le Morte d'Arthur; and 7th Sea, "swashbuckling and sorcery" set in a fantasy 17th-century Europe.
Many of Chaosium's product lines are based upon literary sources. Other notable game designers have written for Chaosium. These include David Conyers, Matthew Costello, Larry DiTillio, David A. Hargrave, Rob Heinsoo, Keith Herber, Jennell Jaquays, Katharine Kerr, Reiner Knizia, Charlie Krank, Robin Laws, Steve Perrin, Sandy Petersen, Ken Rolston, Ken St. Andre, Jonathan Tweet, John Wick, and Lynn Willis; among others.