Dennis Quaid Rocks Out Onstage With The Doors' Robby Krieger

Dennis Quaid's living out a rock 'n roll fantasy ... taking the stage with one of The Doors' last surviving members, Robby Krieger, for a surprise gig at the legendary Whisky a Go Go.

TMZ got this clip of the actor, and obvious Doors fan, stepping into Jim Morrison's shoes to front the band for their classic, "Riders on the Storm."

DQ showed off his pipes to the delight of the crowd inside the WeHo venue -- the event was presented by Sammy's Beach Bar Rum.

The night was dedicated to Mikeal Maglieri, the owner of the Whisky and Rainbow Bar & Grill, who passed away last month.

Dennis has always had a passion for music... playing gigs with his band, The Sharks -- also writing and performing the song "Closer to You" in 1987's flick, "The Big Easy."

Dennis, who's a devout Christian, drew on his faith earlier this year to release, "Fallen: A Gospel Record for Sinners"... saying it was inspired by his Baptist upbringing in Texas.