The exes, who were both presenters at the Golden Globes on Monday night, got into a heated argument at Prince's party at the Beverly Wilshire hotel after the Golden Globes early Tuesday morning, People reports.
The magazine says that they were friendly to each other during the Globes' televised ceremony and even shared laughs at InStyle's annual party, but things turned rotten later on.
According to eyewitnesses on the scene, Timberlake was chatting with actress Jessica Biel (who has been linked with Derek Jeter) when Diaz approached. Biel stepped away, and the former couple had a serious exchange, according to People.
"If that's how she wants to get him back, it won't work," an insider told the Post. "She's desperate."
Diaz walked away, Timberlake followed, and the two were soon spotted in an intense 40-minute talk in a side room, a conversation that the New York Post described as "awkward."
Diaz reemerged from the chat looking composed, People reports, but not before Timberlake, "slammed his fist (down) into a cabinet."
Diaz and Timberlake announced that they had officially split last week.
They issued a joint statement confirming they ended their romantic relationship, saying they "have done so mutually and as friends, with continued love and respect for one another." The couple began dating in 2003.
This isn't the first time Biel has been at the center of a ruckus at a swanky affair.
At the GQ Men of the Year dinner in December, Lindsay Lohan reported caused a ruckus when she spied Biel at the event with her ex-assistant, who quit several months before and apparently left the "Bobby" star furious.
A source told the newspaper that the starlet "flipped out" and started yelling: "If she stays, I'm outta here! I can't look at that girl! I can't believe you would allow an assistant in here — she doesn't belong in here!"