Greyhawk is a rules supplement written by Gary Gygax and Robert J. Kuntz and published for the original edition of the Dungeons & Dragons (D&D) fantasy role-playing game.

The 2004 publication 30 Years of Adventure: A Celebration of Dungeons & Dragons suggested that details of Gygax's Greyhawk campaign were published in this booklet.

[9] Ready Ref Sheets (1977) from Judges Guild collected 20 pages of charts that were taken from the original D&D boxed set, as well as Chainmail and Greyhawk.

[6]: 66  Material from Greyhawk, along with the original D&D and the Blackmoor and Eldritch Wizardry supplements, was revised by J. Eric Holmes for the Dungeons & Dragons Basic Set (1977).

[4] Shannon Appelcline, in his 2011 book Designers & Dragons, considers Greyhawk an "innovation" because at the time "supplements were largely unheard of in the wargaming industry.

Ewalt valued that Greyhawk showed that Dungeons & Dragons was meant to be adapted and evolved by the players beyond the original rules to make it most suitable for each individual gaming group.