Justin Bieber -- Poppin' Bottles For 21st Birthday

Justin Bieber rang in his 21st birthday doing something common -- drinking champagne -- and something very few people can do ... popping that champagne ON A PRIVATE ISLAND!

Sources close to Justin tell us he rented a Caribbean isle near Grenada for the "I'm finally legal" bash ... and based on this video, he had a great time celebrating with friends.

Bieber posted a pic on Shots of the party ... and it looks like he's getting close again with model Yovanna Ventura -- who he was rumored to be hooking up with in the past.

Wonder if she came with a ribbon?