Our spies caught Mr. HSM engaged in some serious PDA with another actress.
Here are all the gooey details...
(Warning: Disney Channel fans might want to stop reading!)
The evening started off innocently enough, as Zac cozied up to Aussie starlet Teresa Palmer at a reception for her upcoming flick Take Me Home Tonight at L.A.'s Pink Taco. They drank and chatted up together before cutting out around 10 p.m.
Then they hightailed over to Voyeur nightclub in West Hollywood. And that, an eyewitness tells E! News, is when things got friskier, as the Zac laid down the mack on the blond beauty.
The twosome drank, dirty danced and made out. In other words, it was not G-rated High School Musical stuff.
"They got there a little after midnight," says a club insider. "They were at a VIP table...They were dancing, standing up by the table and dancing. They all took tequila shots together. He was grabbing her butt and doing very suggestive dancing. Then they made out a couple of times standing up where they were dancing. I was surprised they were doing that in front of everyone. It was like they didn't care.
"They were there for like an hour and left together."
The source adds that the 23-year-old Efron was "all about her" the entire night. The only time she left his side was when skateboarder-snowboarder Shaun White came up and talked to Zac. Otherwise, "they were side by side the entire night."
The up-and-coming Palmer, 24, has had roles in such films as The Grudge 2, December Boys with Daniel Radcliffe and last year's Sorcerer's Apprentice with Nicolas Cage. She also stars in the sci-fi flick I Am Number Four, due out Feb. 18. Take Me Home, which also stars Topher Grace, Anna Faris and Michelle Trachtenberg, comes out March 4. She also has a big role in the upcoming Mad Max: Fury Road.
She has been known to date costars, including Grace and Russell Brand (Bedtime Stories).
In any case, Palmer didn't look the worst for wear this morning, where she attended the press junket for Take Me Home and told E! News that Zac "is such a sweetheart."
Just in time for Valentine's Day...