The Hollywood A-listers have been together since 2005 and have six children together, including three adopted children.
A date has not yet been set. According to the BBC, a spokeswoman for the couple said that the engagement was a "promise for the future" between the couple.
She added that they were "very happy" about the news.
This will be Jolie's third marriage after Jonny Lee Miller and Billy Bob Thornton. For Pitt, it will be his second marriage. He was previously married to Jennifer Aston.
Pitt and Jolie are two of Hollywood's most recognised faces. They met on the set of the film they starred in together, Mr and Mrs Smith.