Fagin (/ˈfeɪɡɪn/) is a fictional character and the secondary antagonist in Charles Dickens's 1838 novel Oliver Twist. Originally depicted by Dickens as explicitly Jewish, in the preface to the novel, he is alleged to be a "crafty old Jew" and a fence. In the story, Fagin is the leader of a group of children (the Artful Dodger and Charley Bates among them) whom he teaches to make their livings by pickpocketing and other criminal activities, in exchange for shelter. A distinguishing trait is his constant and insincere use of the phrase "my dear" when addressing others. At the time of the novel, he is said by another character, Monks, to have already made criminals out of "scores" of children. Nancy, who is the lover of Bill Sikes (the novel's lead villain), is confirmed to be Fagin's former pupil.

Fagin is a confessed miser who, despite the wealth that he has acquired, does very little to improve the squalid lives of the children he guards, or his own. In the second chapter of his appearance, it is shown (when talking to himself) that he cares less for their welfare, than that they do not "peach" (inform) on him and the other children. Still darker sides to the character's nature are shown when he beats the Artful Dodger for not bringing Oliver back; in his attempted beating of Oliver for trying to escape; and in his own involvement with various plots and schemes throughout the story. He indirectly but intentionally causes the death of Nancy by falsely informing Sikes that she had betrayed him, when in reality she had shielded Sikes from the law, whereupon Sikes kills her. Near the end of the book, Fagin is captured and sentenced to be hanged, in a chapter which portrays him as pitiable in his anguish. Though portrayed as cunning and manipulative, Fagin’s characterization has been the subject of longstanding criticism for its antisemitic overtones.

In the first 38 chapters of the original edition of the novel, Fagin is referred to as "the Jew" over 250 times, while being called by his name far less frequently. Dickens notoriously stated that he chose to depict Fagin as Jewish by claiming that, in the novel's period, "the class of criminal almost invariably was a Jew". The novelist later removed many references to Fagin's Jewishness in revised editions of the novel, particularly after being contacted in 1863 by Eliza Davis, the wife of a Jewish banker to whom Dickens had sold his home, though the explicit framing of Fagin as a Jew continued to persist in many unauthorized publications of the novel's original edition.

Fagin has since become a prominent example in literary and cultural studies of the antisemitic stereotype of the "Jewish villain" and as "one of the most infamous antisemitic caricatures of all time." In response to this legacy, creators such as Will Eisner have reinterpreted the character from more sympathetic or critical perspectives, as in Eisner’s 2003 graphic novel Fagin the Jew.

Fagin is portrayed as a criminal mastermind who kidnaps orphaned children and trains them to be pickpockets in return for sheltering and feeding them; he keeps the ill-gotten money for himself. His "wards" include the novel's title character Oliver Twist, the Artful Dodger, Charley Bates, and Nancy. He also trained the novel's main antagonist, housebreaker Bill Sikes, who later became his main source of stolen goods.

Oliver at first believes that Fagin is an artisan who makes wallets and handkerchiefs which are, in fact, stolen at Fagin's order, and goes along with his new teacher's orders. The only one in the gang to protect Oliver is Nancy, who is also Sikes's lover.

After Oliver is arrested for supposedly picking the pocket of a gentleman named Mr. Brownlow (an act actually committed by the Artful Dodger and Charley Bates), Brownlow takes pity on Oliver and brings him to his house, helping him recover from the abuse and malnourishment he has suffered. Fagin and Sikes, fearing that Oliver will betray him to the police, force Nancy to help Sikes kidnap the boy again. Oliver tries to run away, but Sikes almost beats him into submission, stopping only when Nancy begs him to show mercy while the cowardly Fagin tries to smooth matters over between the two.

Fagin forces Oliver to help Sikes burgle a house owned by the wealthy, elderly widow Mrs. Maylie. After breaking into the house, Oliver is shot in the arm and Sikes abandons him while he makes his own escape. Mrs. Maylie and her niece Rose take Oliver in and raise him in a polite society. Fagin later meets with a fellow criminal, the mysterious Mr. Monks, and plots with him to destroy Oliver's newfound reputation.