Once Upon a Time...in Hollywood is a 2019 comedy-drama film written and directed by Quentin Tarantino. Produced by Columbia Pictures in association with Bona Film Group, Heyday Films, and Visiona Romantica, and distributed by Sony Pictures Releasing, it is a co-production between the United States, United Kingdom, and China. It features an ensemble cast led by Leonardo DiCaprio, Brad Pitt, and Margot Robbie. Set in 1969 Los Angeles, the film follows a fading actor and his stunt double as they navigate the rapidly changing film industry with the threat of the Tate murders looming.

Announced in July 2017, it is the first (and currently only) Tarantino film not to involve Bob and Harvey Weinstein, as Tarantino ended his partnership with the brothers following the sexual abuse allegations against Harvey Weinstein. After a bidding war, the film was distributed by Sony Pictures Releasing, which met Tarantino's demands, including final cut privilege. Pitt, DiCaprio, Robbie, Zoë Bell, Kurt Russell and others joined the cast between January and June 2018. Principal photography lasted from June through November around Los Angeles. Once Upon a Time in Hollywood is the final film to feature Luke Perry, who died on March 4, 2019, and is dedicated to his memory.

Once Upon a Time in Hollywood premiered at the 2019 Cannes Film Festival on May 21, 2019, and was theatrically released in the United States on July 26, and in the United Kingdom on August 14. It grossed over $392 million worldwide and received acclaim from critics, although historical accuracies and artists were criticized. The National Board of Review and the American Film Institute named Once Upon a Time in Hollywood one of the top-ten films of 2019, and it won the Golden Globe for Best Picture - Comedy. Once Upon a Time in Hollywood was nominated for ten awards at the 92nd Academy Awards, winning two (Best Supporting Actor for Pitt and Best Production Design), and received numerous other accolades. It has since been considered as one of the best films of the 2010s and 21st century. The Writers Guild of America ranked the film's screenplay the 22nd greatest of the 21st century. Tarantino has stated that Once Upon a Time in Hollywood is his favorite film of those he has made.

A novelization, written by Tarantino in his debut as an author, was published in 2021. A sequel to the film, written by Tarantino, was announced to be in development and set to be directed by David Fincher and distributed through Netflix, with Pitt and DiCaprio in negotiations to return.