Mary-Kate Olsen is two-timing her boyfriend with a surprising new guy – chubby Knocked Up star Jonah Hill, The ENQUIRER has learned exclusively.

The unlikely couple cozied up to one another at Los Angeles’ Château Marmont hotel on Aug. 26 – just one night after the stick-thin actress stepped out to a rock concert with steady beau Nate Lowman.

"It was a shock to see Mary-Kate together with, of all people, geeky Jonah," an eyewitness told The ENQUIRER.

"Mary-Kate and Jonah sat close together and seemed totally into each other. They were both wearing plaid shirts that almost matched. It was the cutest thing."

Actor-writer Jonah, 24, hit it big with the 2007 film Superbad which ushered in a new wave of oddball leading men.

"He’s such an improbable ladies’ man – but when you get to know Jonah, he’s very charming," said an insider.

It’s an out-of-the-blue hookup for Mary-Kate, 22, who has been involved with New York conceptual artist Nate since last fall, and has been romantically linked with Greek shipping heir Stavros Niarchos and the late  Heath Ledger.

Party girl Mary-Kate’s budding romance with Jonah will likely come as a big surprise to 28-year-old Nate, who was photographed embracing her at a Radiohead concert in L.A. less than 24 hours before her intimate dinner with Jonah.

 "I’m sure Nate would have been floored to see the spectacle – the poor guy is being two-timed."

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