Lady Gaga Makes Out with Mystery Man After New Year's Eve Vegas Performance

Lady Gaga may have had the best New Year's Eve of anyone ... gettin' down onstage and off, and -- who's the dude?

Gaga brought the house down in Vegas, performing "Jazz + Piano" at Park Theater. She then beelined it down the strip to NoMad Restaurant and appeared at Brian Newman's "After Dark" show.

In the later appearance, Gaga sang "Fly Me to the Moon," and began a super sexy dance with Newman's wife, former Miss Exotic World winner Angie Pontani.

But then, at the stroke of midnight, Gaga shared a passionate kiss -- not a "friend" kiss by a longshot -- with a dark-haired, bearded dude named Michael. We don't know who he is, but Gaga sure does. She and Michael left hand-in-hand into the Vegas night.