Mordenkainen is a fictional wizard from the World of Greyhawk campaign setting for the Dungeons & Dragons roleplaying game. He was created by Gary Gygax as a player character, only months after the start of Gygax's Greyhawk campaign, and is therefore one of the oldest characters continuously associated with Dungeons & Dragons.

Once Gygax was forced out of TSR, Inc., he lost creative control of Mordenkainen. TSR then made Mordenkainen a powerful wizard with strong convictions against moral absolutes, and the leader of the Circle of Eight, a cabal of eight powerful wizards. In fiction associated with the World of Greyhawk, he has played diverse roles as both protagonist and antagonist.

Official publications for the World of Greyhawk sometimes contradict each other regarding Mordenkainen. It is clear, however, that he is an important figure in the fictional history of the Flanaess.