Credit: Albert Michael/

Cozy costars!

Former High School Musical stars Zac Efron and Ashley Tisdale reunited for a bit of fun in the sun at a Malibu, Calif., beach party Saturday.

Frolicking in the water and on the sand outside The Talent Resources Malibu Beach House as he helped celebrate Tisdale's 26th birthday, Efron, 23, even gave his bikini-clad pal a piggyback ride. Later, the duo cooled off with Heineken Light and VnC cocktails.

"So happy zacary made it to my party!!! He's my brother/best friend!!!" Tisdale Tweeted after the bash. "Had such a great birthday party yesterday! I'm so lucky to have such amazing friends, family, and man in my life!"

PHOTOS: See more pics of Zac and Ashley's fun in the sun at Popsugar

No, the man Tisdale was referring to in her Tweet wasn't Efron -- she's been dating music video director Scott Speer since 2009.

"They are like brother and sister," a source tells Us Weekly of Efron and Tisdale. "They were just palling around, having fun. He would never think of her like that."