The pair met over the Christmas break and have started a romance. It is Pitt’s first serious relationship since he split from Angelina Jolie two years ago.
The two actors were introduced by Theron’s ex-fiancé Sean Penn The Sun reported.
They were seen at the legendary Hollywood hangout the Chateau Marmont over the weekend after attending separate screenings of new films.
A source told The Sun: “They have been casually seeing each other for nearly a month now. They’ve been friends for some time — ironically through Sean — but things have developed.”
“Brad came over to Chateau afterwards, changed his outfit and joined Charlize in a corner of the bar.
“She was on a vodka cocktail while he stuck to mineral water.”
Pitt has reportedly stopped drinking since admitting it was a major source of the problems between him and Jolie.
His other high-profile partners include: Jennifer Aniston, Juliette Lewis, Demi Moore and Gwyneth Paltrow.