John Stamos Performs at L.A. Halloween Party as Zombie Elvis

John Stamos surprised attendees at a Halloween event in L.A.'s Hidden Hills neighborhood last night dressed as a zombie Elvis ... and -- have mercy! -- he knocked it out of the park.

Singing "Good Golly Miss Molly" ... John was accompanied by a full band who were all dressed up for the big spooky celebration.

During the band's rendition of "Honky Tonky Women" ... the "Fuller House" star switched over to the drums.

The former "ER" star wasn't the only one to give a performance last night at the star-studded event.

Not only did the longtime Beach Boys special guest grace the stage ... but Melissa Ethridge also performed -- wearing an inflatable alien abduction getup.