Sean Penn and former flame Petra Nemcova seem to have officially rekindled their romance after being spotted together on a series of dates, sources tell Page Six.

They were spotted together at Thursday night’s star-packed birthday bash for U2 front man Bono at The Spotted Pig in the West Village.

Sources say Penn and Nemcova were “very much” together at the party, where guests included Anna Wintour, Graydon Carter, Ed Burns and Christy Turlington-Burns, Robert De Niro and Grace Hightower, Jay-Z, Chelsea Clinton and Marc Mezvinsky.

We reported last month that Penn and Nemcova, who were first linked in 2008 during Oscar season, had been spotted cozying up to one another at Lexington Bar and Books during a night out.

Their reunion happened weeks after Nemcova called off her wedding to Jamie Belman, whom the former Sports Illustrated swimsuit cover model dated for eight months before getting engaged in January.

In April, a source told us that Nemcova was “all over Penn” while they had dinner, and the two left the restaurant together.

On Thursday, Penn and Nemcova seemed to be trying to keep a lower profile. Onlookers told us that the couple were not overly affectionate, but they were definitely together. A rep for The Spotted Pig had no comment.

Meanwhile, attendees at Bono’s birthday party told us that the highlight of the event was when Bono sang a love song to his wife, Ali Hewson, with a traditional Irish band.

DJ Ani Quinn spun tunes for the crowd, while Julian Schnabel, Naomi Campbell and Vladimir Doronin, looked on as Bono’s daughter Eve Hewson and her friends danced as the party going on until 4 a.m., said onlookers.

Eve co-starred with Penn in the indie movie “This Must Be the Place,” which debuted at the Cannes Film Festival last year. Reps for Penn and Nemcova didn’t get back to us last night.