Mark Zuckerberg Denies Calling Chael Sonnen About Fighting Elon Musk At UFC 300

Chael Sonnen was talking out the side of his neck when he claimed Mark Zuckerberg personally called him to reveal the Elon Musk fight would go down at UFC 300 ... so says a rep for the Meta CEO, who tells TMZ Sports no such phone conversation ever took place.

The former UFC star made the wild declaration on "The MMA Hour" earlier Monday ... saying Zuck dialed him up to share a major update on his proposed fight with the Twitter honcho during his appearance on the show.

"Zuckerberg contacts me because he saw your tweet that I'm coming on," Sonnen said. "Zuckerberg has told me that he has agreed to fight Elon Musk at UFC 300 and it was a very big deal."

Sonnen swore up and down that he was 100% serious ... adding, "That just happened. [My producer] Ethan is my witness, my son, Thero, my dog, Duke. This is a shoot."

While a Zuck vs. Musk fight could very well happen at UFC 300 next year for all we know, a spokesperson for the Facebook co-founder was quick to shut down any talks of a phone chat with the former contender -- saying, "I have confirmed with Mark that he did not call Chael Sonnen."

Things have been picking up steam after the two billionaires traded public jabs all last week ... and UFC honcho Dana White told us both sides are dead serious about fighting.

In fact, Dana thinks it could be the most successful fight in history ... and has the potential to rake in three times the amount of Pay-Per-View buys than Floyd Mayweather and Conor McGregor.