Oldboy (Korean: 올드보이; RR: Oldeuboi; MR: Oldŭboi) is a 2003 South Korean 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 resembling 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 as he seeks revenge against his enigmatic captor (Yoo Ji-tae). His quest becomes tied in with romance when he falls in love with a young sushi chef, Mi-do (Kang Hye-jung).

Oldboy attained critical acclaim and accolades worldwide, including winning the Grand Prix at the 2004 Cannes Film Festival, where it garnered high praise from Quentin Tarantino, the president of the jury. In the United States, film critic Roger Ebert stated 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". The film's action sequences, particularly the single shot corridor fight sequence, also received commendation for their impressive execution.

The film's success led to two adaptations: an unauthorized Hindi remake in 2006 and an official American adaptation in 2013. As part of Park Chan-wook's The Vengeance Trilogy, it serves as the second installment, following Sympathy for Mr. Vengeance (2002) and preceding Lady Vengeance (2005).

The film is regarded as one of the greatest films of all time and has been included in numerous "best-of" lists by many publications. In 2008, Oldboy was placed 64th on an Empire list of the top 500 movies of all time. In 2020, The Guardian ranked it number 3 among the classics of modern South Korean Cinema.