And the couple was introduced by her former fiancé Sean Penn.
The Sunday Sun newspaper is reporting that sparks are flying between the two A-listers.
According to sources the pair were all over each over at a showbiz party last week.
Their relationship, which began over Christmas, is Pitt's first serious romance since he split from Angelina Jolie back in 2016.
Oscar winner Theron, who called off her marriage to Penn in 2015, is understood to have visited Brad at his home in the Loz Feliz area of Los Angeles.
But she has not, as yet, met his six children with Jolie.
Both Pitt, 55, and Theron, 43, are both known for their romances with other high-profile stars.
A source said: "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."
Last weekend the pair were spotted together after separate Saturday-night movie screenings.
Pitt went to watch 'If Beale Street Could Talk' — which he is exec producer of — at a private house in the Hollywood Hills.
Theron was special guest at a showing of 'Roma' at the historic Chateau Marmont Hotel.
The source added: "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.
"They were ridiculously touchy-feely and his arm was around her back. At one point he winked at her.
"Brad seemed in a really good place — they both looked really happy."
Pitt quit alcohol after admitting his drinking was the main problem in his marriage to Jolie.
The pair has been embroiled in a bitter divorce that is expected to get resolved soon.
The actor famously dated Gwyneth Paltrow before marrying first wife Jennifer Aniston.
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