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

Christmas came early for Slash fans who attended the guitarist’s star-studded show in New York on Thursday, where he delivered an eclectic covers set with help from Yungblud, Bruno Mars, Eddie Vedder ...

With a band ft members of Guns N' Roses and Red Hot Chili Peppers, Andrew Watt hosted Eddie Vedder, Bruno Mars, & more at Capitol Theatre.

Check out this clip from Thursday's private show at the Capitol Theatre in Port Chester, NY ... Bruno is on stage with Slash, ...