John Travolta dresses up as Danny Zuko for Grease sing-a-long screening

Grease star John Travolta donned his old costume from the 1978 movie musical to surprise fans at a Hollywood sing-a-long screening.

The star played teenage 1950s bad boy Danny Zuko in Grease, with Olivia Newton-John starring as his good girl love interest Sandy Olsson.

At the Hollywood Bowl event on Friday, Travolta joined director Randal Kleiser and castmembers Barry Pearl, Didi Conn, Kelly Ward and Michael Tucci - all of whom were apparently unaware he was in attendance before he walked on stage.

A video posted to Travolta's Instagram page showed him emerging from the crowd while dressed as the leather-clad greaser hero, resulting in huge cheers.

"Tonight at the Hollywood Bowl, for the first time I surprised everyone at the 'GREASE' Sing-A-Long and dressed up as Danny Zuko," he captioned the post. "No one knew. Not even the cast. Thank you for a great evening."

While on stage, he said, "L.A! I thought you were going back to Australia?," in reference to a line from the film. "That's cool, that's cool, baby. You know, rocking and rolling and whatnot," he continued.

The Pulp Fiction star and the rest of his former castmates then sang "a-womp-bop-a-looma a-womp-bam-boom" before he conluded, "Enjoy the show. We love you!"