The Man Who Killed Don Quixote is a 2018 adventure-comedy film directed by Terry Gilliam and written by Gilliam and Tony Grisoni, loosely based on the 1605/1615 novel Don Quixote by Miguel de Cervantes. Gilliam tried to make the film many times over 29 years, which made it an infamous example of development hell.

Gilliam started work on the film in 1989 but was unable to secure funding until 1998 when it entered full pre-production with a budget of $32.1 million without American financing, with Jean Rochefort as Quixote, Johnny Depp as Toby Grummett – a 21st-century marketing executive thrown back through time – and Vanessa Paradis as the female lead. Shooting began in 2000 in Navarre, but difficulties such as sets and equipment being destroyed by flooding, the departure of Rochefort due to illness, problems obtaining insurance for the production and other financial difficulties led to a sudden suspension of the production and its subsequent cancellation. The original production was the subject of the documentary film Lost in La Mancha, which was intended to be a making-of but was released on its own in 2002.

Gilliam’s repeated attempts to relaunch production between 2003 and 2016 included Depp, Ewan McGregor and Jack O'Connell as Toby and Robert Duvall, Michael Palin and John Hurt as Quixote. All were cancelled for various reasons, such as failing to secure funds, Depp's busy schedule and eventual loss of interest in the project and Hurt being diagnosed with cancer that eventually led to his death. After yet another failed attempt, it was unexpectedly reported in March 2017 that filming had finally started, with Adam Driver, who was confirmed as Toby in 2016 and helped secure funding, and Jonathan Pryce, who had been part of the original 2000 production in a different role, as Quixote. On 4 June 2017, Gilliam announced that the shooting of the film was complete, 17 years after it originally started. The final version of the film is set in modern-day rural Spain and now features Toby as a director shooting commercials and "Quixote" as an insane shoemaker convinced that he is the character from Cervantes' novel.

The Man Who Killed Don Quixote premiered on 19 May 2018, simultaneously acting as the closing film at the 2018 Cannes Film Festival and being released in French theaters. Gilliam faced difficulties in the process of releasing the film worldwide, partially due to a lengthy legal dispute with former producer Paulo Branco. Subsequently, the film was only released in a few other countries in 2018; it was released in the United States and Canada on 10 April 2019 and in the United Kingdom and Ireland on 31 January 2020. It received mostly positive reviews from critics, who praised its creativity and Driver and Pryce's performances but found it "messy", and was a commercial failure, earning $2.5 million against a budget of €16 million, plus its original 2000 $32 million budget. A follow-up to Lost in La Mancha, titled He Dreams of Giants and following the making of the final version of The Man Who Killed Don Quixote, was released in 2019.