The film beat out "Dolemite Is My Name", "Jojo Rabbit", "Knives Out" and "Rocketman" for the honour.
Producer David Heyman accepted the prize on the stage, after being put in front by director-writer Tarantino, Margot Robbie and other team members. The film's leading stars Leonardo DiCaprio and Brad Pitt remained seated.
"Quentin is nothing if unpredictable," Heyman said, adding: "A few seconds ago he told me I was going to speak."
Heyman thanked everyone who contributed to the film on-screen and off.
"Also a big thanks to maestro, Mr. Quentin Tarantino. Quentin said before we started filming, 'I want you to have such a good time on this film that the next will be miserable'. He wasn't wrong. Thank you, Quentin," he said.
The movie follows the fictional actor Rick Dalton (DiCaprio) as he navigates 1960s Hollywood, while trying to stay relevant as an actor. Pitt is seen as his stunt double and close friend, while Margot Robbie plays Sharon Tate. Tarantino has woven real life stories into the plot with the infamous Charles Manson murders, including actress Sharon Tate, as a backdrop to make it more intriguing.
Tarantino also won Best Screenplay - Motion Picture. He was pitted against Noah Baumbach ("Marriage Story"), Bong Joon-ho and Han Jin-won ("Parasite"), Anthony McCarten ("The Two Popes") and Steven Zaillian ("The Irishman").
He dedicated his win to Robert Bolt, "the dean of screenwriters". He also congratulated himself for solely writing the script of the film as he said "I did it".
The director went on to thank the film's "fantastic" cast, saying that they took the script "from the page and added a slightly different layer from the pagea.
Giving shout-outs to Pitt, DiCaprio, Julia Butters and Margot Robbie, he said that they brought "more goodness than a movie I had ever been involved in".
( With inputs from IANS )