Justin Bieber -- Birthday Partying Bahamas Style

Justin Bieber dropped tens of thousands of dollars to celebrate his 20th birthday with family and friends at a secluded beach resort in the Bahamas ... TMZ has learned.

Sources close to Bieber tell us Justin checked into The Cove -- a 5-star all-inclusive Caribbean resort on the island of Eleuthera -- with his mom, dad, siblings and several close friends, including Khalil and Lil Za.

We're told ... each villa goes for $4,275 a night and they're equipped with private butlers, private pools, and full kitchens -- and Bieber rented several.

Biebs and his crew flew to the Bahamas Friday ... then sailed to The Cove on a private yacht. We're told he celebrated his birthday Saturday on the boat. On Sunday, they had a picnic on the beach.

Back at his crib in Atlanta, fans celebrated his birthday, too ... decorating the gate outside his rented home with balloons, posters and personal messages.