Bruno Mars Performs 'Smells like Teen Spirit' With Slash, Duff McKagan, Chad Smith

Here's Bruno Mars in a state of nirvana ... he's performing one of Nirvana's biggest hits ... and it's all on video.

Check out this clip from Thursday's private show at the Capitol Theatre in Port Chester, NY ... Bruno is on stage with Slash, Duff McKagan, and Chad Smith when they start playing Nirvana's 1991 classic, "Smells Like Teen Spirit."

Bruno's into it, and so is Orlando Bloom ... he's the cameraman here who recorded the video and shared it on his Instagram story. Thanks, Orlando!

Eddie Vedder, Anthony Kiedis, Brandi Carlile, and Yungblud also hit the stage for the exclusive event -- a show put on by Eldridge Industries -- and they blended pop, punk, grunge, and classic rock in an epic jam sesh.