Justin Bieber -- Underage Clubbin' in NYC

Justin Bieber broke out his finest party tank top and hit the NYC club scene HARD last night -- rollin' to 3 hot spots in a matter of hours ... problem is, he's not 21.

Cameras rolled as Bieber and his posse rolled into Avenue nightclub (where Lindsay Lohan allegedly punched a gypsy) ... then to Marquee ... followed by 1 Oak.

Here's the issue ... we checked into the age requirements to hit the different venues and we found Marquee had advertised that Wednesday was a 21+ night.

Bieber is only 19.

It's possible Marquee has a license to allow minors into the venue ... and they CHOSE to make it a 21+ night, which would mean no laws were broken. We're checking into that ...

Ditto for Avenue ... which also advertised Wednesday as a 21+ evening.

As for 1 Oak -- we're diggin' on that.