Pitt's manager Cynthia Pett-Dante confirmed the news after Jolie was seen wearing an engagement ring at an event Thursday night at the Los Angeles County Museum of Art.
There isn't a wedding date at this time.
Pitt, 48, and Jolie, 36, have three adopted children and three biological children. Pett-Dante said "Their kids are very happy." Pett-Dante also told Eyewitness News that Pitt helped design the ring.
Pitt and Jolie have been partners since they began a relationship while working on the 2005 film "Mr. & Mrs. Smith."
Jolie was briefly married to Billy Bob Thornton, but they divorced in 2003. She was also married to British actor Jonny Lee Miller before they divorced in 1999.
Pitt was previously married to actress Jennifer Aniston, but they divorced in 2005 after five years of marriage.
The Associated Press contributed to this report.