Sean Penn is at the center of a sensational cheating scandal that could torpedo his romance with Hollywood glam girl Charlize Theron!

Sean, 54, and the “Prometheus” star, 39, went public with their relationship earlier this year and were said to already be talking marriage.

But wedding bliss was the farthest thing from the serial womanizer’s mind when he checked into Hollywood’s Sunset Tower Hotel with another woman on Nov. 14 at around 10:30 p.m., according to an eyewitness, who is coming forward to The National ENQUIRER.

His squeeze on that night was a “hot” young woman who appeared to be in her mid-20s, said the source.

“You could tell from their body language that they looked like they were hooking up,” the insider noted. “Sean Penn doesn’t hold hands with just anyone!

“He (Sean) looked nervous when he saw me next to him at the front desk, so he started to ask the attendant to hurry to give him his key.

“He walked to the elevator and pressed the button several times like he was in a hurry. Then they got in to go upstairs.”

Dressed in a black suit and white shirt, Sean doted over the “mystery” woman who was “tall and long-legged with short hair and wore a short black dress,” according to our spy. “It looked like there was a lot of sexual chemistry between them.

“He was holding her hand real tight. I don’t think they were going up to the room to play chess, that’s for sure!”

When asked for comment about his apparent fling, a source close to Sean insisted the woman alongside him was indeed Charlize.

And Charlize's rep confirmed it was her.

Adding to the relationship drama, another source told The ENQUIRER that a worried Charlize already fears Sean – a Best Actor Oscar winner for 2003’s “Mystic River” and the 2008 drama “Milk” – will stray, but not with a younger woman!

Charlize has confided to friends that she fears Sean would ditch her to reconnect with ex-wife Robin Wright, now that she’s called off her engagement to 34-year-old actor Ben Foster!

“Charlize has every reason in the world to worry about Sean straying,” a close friend revealed.

“He’s done it before, and he’ll probably do it again. That’s just in his nature. He’ll never change, no matter how much she might hope!”

Warned Tia Brown, a New York–based relationship expert: “Unfortunately, it doesn’t matter why Sean Penn went upstairs – it could very well be a meeting – but it simply doesn’t look good.”