WATCH out, CHARLIZE! Your new beau, SEAN PENN, may be just using you to get back at his ex-wife!

Penn, 53, has gotten hot and heavy with 38-year-old South African beauty Charlize Theron – but a source says he may have done it only to get even with his ex-wife, Robin Wright, after she hooked up with her much younger “Rampart” co-star Ben Foster, 33.

“Sean has been seething with jealousy ever since Robin got together with Ben in 2011,” said a source. “And he went nuclear in December when he heard that they were engaged.”

According to the source, Sean – whose hot-headed ways and reported womanizing led to his split with Robin in 2010 – can’t stand the idea of her finding happiness with a man 14 years her junior and 20 years younger than him.

“For the first time in ages, Robin feels happy and settled,” an insider recently told The ENQUIRER. “She went through hell with Sean’s mood swings and violent temper, and she never thought she’d find love again. But Ben worships her and gets along great with her kids.”

Sean and 47-year-old Robin – who nabbed a Best Actress Golden Globe for her role on the Netflix series “House of Cards” – married in 1996 and have two grown children together.

“Sean believes that his public ro­mance with Charlize – who looks a lot like a younger Robin – will truly get under her skin,” said the source.

But Charlize may be the one that’s going to be hurt by Sean’s revenge romance scheme. The Oscar-winning actor – who’s recently dated a string of stunning women, including actress Scarlett Johansson, model Petra Nemcova and rocker Florence Welch – has long had a reputation for having a wandering eye.

Meanwhile, Charlize has only been linked with “True Blood” star Alexander Skarsgard since her split from longtime love Stuart Townsend in 2010. The main man in her life in recent years has been her 2-year-old son, Jackson, whom she adopted as a newborn in 2011.

Now, the blonde beauty has “fallen hard and fast for Sean,” said the source. “He’s been nothing but charming so far. But if things stay true to form, Charlize will get burned by Sean.

“Charlize really believes that Sean is going to change his ways for her. But he hasn’t even spent any time with Jackson yet, and no one else is expecting bad-boy Sean to suddenly reform."