Greyhawk, also known as the World of Greyhawk, is a fictional world designed as a campaign setting for the Dungeons & Dragons fantasy roleplaying game. Although not the first campaign world developed for Dungeons & Dragons—Dave Arneson's Blackmoor campaign predated it by about a year—the world of Greyhawk closely identified with early development of the game beginning in 1972, and after being published it remained associated with Dungeons & Dragons publications until 2008. The world itself started as a simple dungeon under a castle designed by Gary Gygax for the amusement of his children and friends, but it was rapidly expanded to include not only a complex multi-layered dungeon environment, but also the nearby city of Greyhawk, and eventually an entire world. In addition to the campaign world, which was published in several editions over twenty years, Greyhawk was also used as the setting for many adventures published in support of the game, as well as for RPGA's massively shared Living Greyhawk campaign from 2000 to 2008.

Dungeons & Dragons is a fantasy tabletop role-playing game (RPG) originally created and designed by Gary Gygax and Dave Arneson. The game was first published in 1974 by Tactical Studies Rules, Inc. (TSR). It has been published by Wizards of the Coast, later a subsidiary of Hasbro, since 1997. The game was derived from miniature wargames, with a variation of the 1971 game Chainmail serving as the initial rule system. D&D's publication is commonly recognized as the beginning of modern role-playing games and the role-playing game industry, and also deeply influenced video games, especially the role-playing video game genre.