Sean Penn and Charlize Theron made their relationship public the night before the Golden Globes. Theron was by “The Secret Life of Walter Mitty” actor’s side at his Help Haiti Home gala Saturday night at the Montage Hotel, where U2 were surprise musical guests.

After Penn and Theron had been spotted at his Hawaii home over the holidays, insiders wondered whether he’d bring her to the gala. They didn’t appear on the red carpet together, but inside, Theron was seated next to Penn, and the two kissed and held hands.

Penn even revealed that Theron convinced him to give up his gun collection, telling the crowd that “a strong woman who happens to be from South Africa” changed his mind about owning guns. As a teenager, Theron witnessed her mother shoot and kill her own father to protect her.

Penn said that “considering how liberating of bull[bleep] and ugliness it would be” to “get rid of the guns,” Jeff Koons agreed to make a sculpture of all Penn’s 65 firearms. The work was put up for auction, with Anderson Cooper just outbidding his CNN colleague, the outspoken anti-gun Piers Morgan, to buy it for $1.4 million.

U2 played a three-song set to close the benefit for guests including Julia Roberts, Gwyneth Paltrow and Chris Martin and Michael Douglas. Bono quipped from the stage: “It’s been a while since we played a hotel lounge . . . the last time was a bar mitzvah.” The night raised $6 million for Penn’s J/P Haitian Relief Organization.

Meanwhile, at Sony’s pre-Globes party at the Beverly Hills Hotel, Jack Nicholson was schmoozing with Adam Sandler, Lorne Michaels and Jimmy Fallon, while Jennifer Lawrence gabbed with Tom Hanks, who was overheard saying that “the greatest thing in life is his grandchildren.” The “Captain Phillips” and “Saving Mr. Banks” star stayed till the bitter end and obliged guests of the hotel with photos as he waited for his car with wife Rita Wilson.

Spies said that at last call at the hotel’s Polo Lounge, Cate Blanchett was seen hanging at a table that included Brett Ratner — while Michaels and Fallon closed down the place with “french fries and vodka.”