Oldboy (Korean: 올드보이; RR: Oldeuboi; MR: Oldŭboi) is a 2003 South Korean neo-noir action thriller film directed and co-written by Park Chan-wook. A loose adaptation of the Japanese manga of the same name, the film follows the story of Oh Dae-su (Choi Min-sik), who is imprisoned in a cell which resembles a hotel room for 15 years without knowing the identity of his captor or his captor's motives. When he is finally released, Dae-su finds himself still trapped in a web of conspiracy and violence. His own quest for vengeance becomes tied in with romance when he falls in love with an attractive young sushi chef, Mi-do (Kang Hye-jung).

The film won the Grand Prix at the 2004 Cannes Film Festival and high praise from the president of the jury, director Quentin Tarantino. The film has been well received by critics in the United States, with film critic Roger Ebert stating that Oldboy is a "powerful film not because of what it depicts, but because of the depths of the human heart which it strips bare". It also received praise for its action sequences, most notably the single shot fight sequence. It has been regarded as one of the best neo-noir films of all time and listed among the best films of the 2000s in several publications. The film has been remade twice, an unauthorised 2006 Hindi film and a 2013 American film. The film is the second installment of Park's The Vengeance Trilogy, preceded by Sympathy for Mr. Vengeance (2002) and followed by Lady Vengeance (2005).