Justin Bieber Performs, Dances with Fan at Billionaire Wedding Celebration

Justin Bieber put on one heck of a show at his most recent concert ... no, he's not touring again -- he performed at a ceremony for a billionaire's wedding.

The singer-songwriter arrived in Mumbai yesterday for Anant Ambani and Radhika Merchant's sangeet, a pre-wedding ceremony filled with singing and music ... and, instead of DJ, the couple brought the Biebs!

Check out the clip ... JB hits the floor, dancing by himself before pulling a girl up and having her twirl a couple times in front of the crowd.

More video is out online too BTW ... featuring Justin singing classic tracks like "Baby" and getting the crowd jumping -- definitely a wedding celebration these guests won't soon forget.

Of course, the happy couple's well-known at this point for having some of the world's biggest stars in attendance ... remember, back in March, Rihanna performed at a pre-wedding ceremony for Merchant and Ambani -- so the star power has stayed high throughout the long pre-wedding.

As for Bieber ... it's a rare performance. He's appeared onstage sporadically since canceling his "Justice" tour after he revealed he was diagnosed with Ramsay Hunt Syndrome.

His last big gigs have been a guest spot at Coachella and a show during NHL All-Star Game Weekend in Toronto ... so, only the biggest stages get the guy back on the mic live.