The couple split during the weekend following a series of arguments and the actress attended Monday's Costume Institute Gala at the Metropolitan Museum of Art in Manhattan by herself.
"Sofia was confiding in friends at the ball that she and Nick have split," an insider told the Post. "They have been fighting a lot recently and have been on the brink of splitting many times. It is not yet clear if they are over for good, but they are done as a couple for now."
The couple met at a Golden Globes party in 2010, the newspaper said.
Ray Liotta sues skin care company over use of likeness
Aaron Carter is still in love with Hilary Duff