EXCLUSIVE

Justin Bieber wasn't born with the talent to become an international superstar ... he inhaled it ... in the form of miraculous marijuana smoke -- so says legendary weed icon Tommy Chong.

The "Cheech & Chong" star was out in L.A. yesterday when we asked if he thought more highly of Bieber now that he knows the singer smokes pot -- and Tommy laughed, saying, "Of course!"

He said, "I knew [Bieber] was smoking before he knew it. Because of his music. Because of his dancing. The guy is a phenom ... It's not just his looks. It's his talent. And you don't get talent like that by drinking alcohol. The herb, man! The herb!"

Can't blame Bieber either -- Tommy says all the celebs are smoking ganja these days ... "Weed is mainstream as can be."