Justin Bieber is either true to his art or flush with cash, because he will not turn into a wedding singer under any circumstances.

Justin was leaving Mastro's in Bev Hills Friday night, when our photog asked about possible wedding gigs.  Justin was clear ... no way, no how.

Fact is ... the way the record biz is going, it's a good way to make a fast buck.  Jennifer Lopez performed a wedding a few years back for an oil tycoon and pocketed a cool mil.   Nick Jonas got $100K for singing 3 songs at a Bat Mitzvah.

Justin will, on the other hand, sing for free when a dude proposes to his girl at a mall.