George Lewis Costanza is a fictional character in the United States–based television sitcom Seinfeld (1989–1998), played by Jason Alexander. He has variously been described as a "short, stocky, slow-witted, bald man" (by Elaine Benes and by Costanza himself), "Lord of the Idiots" (by Costanza himself), and as "the greatest sitcom character of all time". He is friends with Jerry Seinfeld, Cosmo Kramer, and Elaine Benes. George appears in every episode except for "The Pen" (third season). The character was originally loosely based on Seinfeld creator Larry David, but named after Jerry Seinfeld's real-life friend Mike Costanza.

Although not a main focus of the show, various details about George's life are revealed throughout. Though never explicitly stated, he grew up in Queens, New York; however in the episode "The Junior Mint" he alludes to growing up in Brooklyn, New York, for at least some time and mentions being from Long Island in "The Nose Job". He met Jerry Seinfeld in a middle-school locker room, and they remained friends from that point on (though it is also said by George that he and Jerry have been best friends "since fourth grade" (The Betrayal). He and Seinfeld attended John F. Kennedy High School, and then they both went on to Queens College. The series starts some years after he graduates.

George is neurotic, self-loathing, and dominated by his parents, Frank and Estelle. He has been best friends with Jerry since their middle-school years. Despite being billed at times as "slow witted", George starts out as a moderately intelligent character—at one point, he mentions an intellectual interest in the American Civil War and, in some early episodes, appears almost as a mentor to Jerry—but gets less sophisticated, to the point of being too lazy even to read a 90-page book.