Jolie, Pitt and their son Pax attended a private viewing at the Los Angeles County Museum of Art on Wednesday, and the Oscar-winning actress was wearing the large, rectangular diamond ring on her left hand.
Pitt's manager confirmed the news to The Associated Press on Friday, after Jolie had been photographed wearing a large diamond ring on her engagement finger.
Cynthia Pett-Dante said that there is not a wedding date "at this time." She also said that the engagement "is a promise for the future."
Pitt and Jolie have been a couple since 2005, and are raising six children – three adopted and three biological – together, but they have never married.
Ms Pett-Dante also said that "their kids are very happy.
A spokesman for jeweller Robert Procop told Us Weekly: "I can confirm that, yes, Robert Procop did indeed diesgn an engagement ring for Angelina Jolie, designed in collaboration with Brad Pitt."
Pitt was previously married to Jennifer Aniston from 2000 to 2005, while Jolie has been married to Jonny Lee Miller from 1996-1999 and Billy Bob Thornton from 2000-2003.
The couple, who met on the set of Mr and Mrs Smith in 2005, are reportedly in discussions to once again work together. Jolie is being eyed by Ridley Scott for the lead female part in thriller 'The Counselor', written by Cormac McCarthy. Pitt is also reportedly lined up for a role in the film.
The Family
Angelina Jolie and Brad Pitt with their six children