George Louis Costanza is a fictional character in the American television sitcom Seinfeld (1989–1998), played by Jason Alexander. He is a short, stocky, balding man who struggles with numerous insecurities, often dooming his romantic relationships through his own fear of being dumped. He is also relatively lazy; during periods of unemployment he actively avoids getting a job, and while employed he often finds ingenious ways to conceal idleness from his bosses. He is friends with Jerry Seinfeld, Cosmo Kramer, and Elaine Benes. George and Jerry were junior high school friends (although in "The Betrayal", Season 9, Episode 8, George says the two have been friends since fourth grade) and remained friends afterward. George appears in every episode except "The Pen" (third season).

The character was based on Seinfeld co-creator Larry David but is surnamed after Jerry Seinfeld's real-life New York friend, Michael Costanza. Alexander reprised his role in an episode of Comedians in Cars Getting Coffee, reuniting with Jerry Seinfeld and Wayne Knight (also reprising their roles as Jerry and Newman, respectively).

George is a son of Frank, an Italian American, and Estelle Costanza, who is Jewish. George twice mentions that he has a brother. Lloyd Braun is a childhood nemesis who George feels was the son his parents always wanted. George's best friend Jerry Seinfeld described Frank and Estelle as "psychopaths", and said in "The Chinese Woman" that, if they had divorced when George was young, he "could have been normal".

In "The Junior Mint", George states he grew up in Brooklyn, New York, where he went to a public school. In a previous episode he mentions he went to high school on Long Island. He met Jerry during his youth, and they remained friends from that point on. George and Jerry both attended John F. Kennedy High School, class of 1971. During their high school years, George and Jerry frequently hung out at a pizzeria called Mario's Pizzas, where the former, having the highest score "GLC", would play Frogger (although Frogger debuted in 1981, well after the pair's high school graduation in 1971). George was picked on by his gym teacher Mr. Heyman, who deliberately mispronounced his name as "can't stand ya" and gave him wedgies. He and Jerry then attended Queens College.

Two of George's cousins appear on the show: Shelly, who briefly appears to visit Estelle in the hospital in "The Contest", and Rhisa, whom George plans to date in order to shock his parents in "The Junk Mail". George talks to his parents about his family in "The Money", during which it is revealed that he had an "Uncle Moe", who "died a young man" and an "Aunt Baby", who died at the age of seven of internal problems. It is also revealed that his mother has a "Cousin Henny". In "The Doll", it is revealed that Frank Costanza was born in Italy and has a cousin, Carlo, who still lives there. As of "The Robbery", George had living grandparents whom he had recently visited, although it is never made clear whether these were his maternal or paternal grandparents.

George is neurotic, self-loathing, mostly selfish, and dominated by his parents, yet also prone to occasional periods of overconfidence that invariably arise at the worst possible time. Throughout Seinfeld's early seasons, despite doing poorly on his SATs and being afraid of embarrassing himself on an IQ test (as depicted in "The Cafe"), George is depicted as moderately intelligent – he mentions interests in the Civil War and musical theatre, and in some early episodes appears almost like a mentor to Jerry – but becomes less sophisticated, to the point of being too lazy even to read a 90-page book (Breakfast at Tiffany's), preferring to watch the movie adaptation at a stranger's house instead. In "The Abstinence", it is discovered that George has what would appear to be genius-level intelligence but can never access it because his mind is always so completely focused on sex. One Chicago Tribune reviewer noted that, despite all his shortcomings, George is "pretty content with himself".

George exhibits several negative character traits, among them dishonesty, insecurity, anxiety, and extreme frugality, many of which seem to stem from a dysfunctional childhood with his eccentric parents Frank and Estelle, and often form the basis of his involvement in various plots, schemes, and embarrassing social encounters. George's extremely narcissistic parents only accept things from George when events revolve around them, and George is blind to see that at the same time his parents treat him like a second grade child. Episode plots frequently feature George manufacturing elaborate deceptions at work or in his relationships to gain or maintain some slight or imagined advantage or (pretend) image of success. He is shown to have an intense fear of commitment. He had success in "The Opposite", where on Jerry's advice he starts to do the complete opposite of what his instincts tell him to do, which results in him getting a girlfriend and a job with the New York Yankees. His anxiety is also evident in "The Note", where he begins doubting his sexuality after he receives a massage from a male masseur.

George refers to himself in the third person (a habit known as illeism) when under extreme stress (e.g. "George is getting upset!"), after befriending a person with a similar trait in "The Jimmy".