Two big stars, one stage.

Jeff Goldblum made a surprise on-stage appearance during Woody Allen's show at Café Carlyle on Monday night, joining the famed director for a rendition of "Sweet Georgia Brown."

We're told Goldblum had tickets to the show, but it was "not planned" that he'd join Allen on stage. Goldblum was on piano, while Allen played clarinet.

"The crowd loved it," a witness said.

Goldblum plays his own run at the Upper East Side venue through Monday.