Infidelity is a central theme in The Great Gatsby, highlighting the moral decay and betrayal among the characters. The novel explores unfaithfulness in marriage and the broader sense of betrayal in personal relationships and societal obligations. Tom Buchanan's affair with Myrtle Wilson and Daisy Buchanan's relationship with Jay Gatsby exemplify the breakdown of marital fidelity. The theme extends beyond romantic relationships, encompassing deceit and the violation of societal norms.

Tom Buchanan's infidelity is evident early in the novel. Jordan Baker casually remarks, "Tom's got some woman in New York," indicating his affair with Myrtle. Tom's blatant disregard for his marriage is further shown when he introduces Myrtle to Nick, saying, "I want you to meet my girl." Myrtle, too, is unfaithful to her husband, Wilson, boldly flirting with Tom in front of him. Her disdain for her marriage is clear when she says, "I married him because I thought he was a gentleman... but he wasn't fit to lick my shoe." The affair between Tom and Myrtle is marked by a lack of respect for marriage, culminating in a violent confrontation when Tom breaks Myrtle's nose for mentioning Daisy's name.

Daisy Buchanan's infidelity is more complex. Her relationship with Gatsby is rooted in past love and current dissatisfaction with her marriage to Tom. Gatsby's insistence that Daisy "never loved" Tom and "only married [him] because I was poor" reveals the emotional betrayal in their marriage. Gatsby's pursuit of Daisy, despite her marital status, shows his disregard for societal norms and moral obligations. His wealth, acquired through illegal activities, further underscores his infidelity to societal expectations.

Beyond romantic infidelity, the novel explores deceit and betrayal in other forms. Gatsby's life is built on lies about his past and wealth, creating a persona to win Daisy's love. This deception is a form of infidelity to the truth, leading to Daisy's eventual disillusionment. Jordan Baker, accused of cheating in a golf match, represents infidelity in sportsmanship and honesty. Nick Carraway, the narrator, claims to "reserve all judgments," yet frequently judges others, highlighting his own unfaithfulness to his stated principles. These layers of infidelity illustrate the pervasive dishonesty and moral decay in the world of The Great Gatsby.