Greg Costikyan, also known as Designer X, is an American game designer and science fiction writer. Costikyan's career spans nearly all extant genres of gaming, including hex-based wargames, role-playing games, boardgames, card games, computer games, online games and mobile games. Several of his games have won Origins Awards. He currently CEO of Manifesto Games, a startup he co-founded with Johnny Wilson in 2005.

Costikyan lives in New York City with his two daughters and is a 1982 graduate of Brown University. He is a frequent speaker at game industry events including the Game Developers Conference and E³.