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This is the 56-year-old's second Oscar win (his first was for Best Picture for 12 Years a Slave) and at the end of his emotional speech he dedicated it to 'my kids, who colour everything I do,' adding: "I adore you."
Pitt played Leonardo DiCaprio's hilarious sidekick in Quentin Tarantino's latest blockbuster, which actually turned out to be his highest grossing yet.
Once Upon a Time in Hollywood is set in 1969 and centres on former TV actor Rick Dalton (DiCaprio) and his stunt double Cliff Booth (Pitt), who move to LA in an attempt to crack Hollywood.
The pair end up moving in next door to Sharon Tate (played by Margot Robbie), the wife of director Roman Polanski, who was infamously killed in the Manson Family murders.
In the film Charles Manson is played by a very creepy-looking Damon Herriman.
Congratulations to Brad Pitt on his #Oscars win for Best Supporting Actor for #OnceUponATimeInHollywood! pic.twitter.com/tyQaPxTjvb
- Once Upon a Time in Hollywood (@OnceInHollywood) February 10, 2020
This win comes off the back of Brad's triumph for the equivalent award at January's Golden Globes where he paid tribute to his co-star with a cheeky Titanic joke. He said: "He's an all-star, he's a gent... Still, I would've shared the raft."
The critically acclaimed movie also picked up the award for Best Motion Picture at the Golden Globes.
In a statement released last year, Tarantino said: "I've been working on this script for five years, as well as living in Los Angeles County most of my life, including in 1969, when I was seven years old.
"I'm very excited to tell this story of an LA and a Hollywood that don't exist anymore. And I couldn't be happier about the dynamic teaming of DiCaprio and Pitt as Rick and Cliff."
Once upon a Time... in Hollywood has been nominated for 10 awards at the 92nd Academy Awards which is being held at the Dolby Theatre in Los Angeles.
The other nominations include Best Picture, Best Director, Best Original Screenplay and Best Actor.
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