Deadline.com reported the casting news and title Wednesday.
Set in 1969 Los Angeles, the Sony Pictures film is set for theatrical release Aug. 9, 2019.
DiCaprio, who worked with Tarantino on 2012's Django Unchained, will play Rick Dalton, former star of a TV western, in Tarantino's next picture.
Pitt, who starred in Tarantino's 2009 movie Inglourious Basterds, will play Dalton's longtime stunt double Cliff Booth in Once Upon a Time in Hollywood.
"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 & Pitt as Rick & Cliff," Tarantino said in a statement to The Hollywood Reporter.
Tarantino is an Oscar-winning filmmaker whose credits include Reservoir Dogs, Pulp Fiction, Jackie Brown, Kill Bill and The Hateful Eight.