Justin Bieber -- Ringside for BFF's Boxing Debut ... Lil Za Wins!!!!

Justin Bieber was part of ANOTHER boxing entourage last night ... to watch his BFF Lil Za get in the ring during a college fight night in Los Angeles ... and TMZ Sports has the footage.

Bieber -- who's usually in Floyd Mayweather's corner -- sat in the front row and held up a sign saying, "WE LOVE YOU ZA" ... as JB's posse member made his boxing debut.

Once the bell rang, Za actually held his own ... and looked PRETTY GOOD as he traded blows with his opponent at the gym at Valley College.

Ultimately, Za WON the fight by decision ... and even got some props from pro boxing superstar Andre Berto, who happened to be in the crowd.