Justin Bieber -- Straight Booze, No Chaser ... with Rae Sremmurd (VIDEO)

Justin Bieber was boozing like a college kid during a Rae Sremmurd show ... taking a huge swig straight from a bottle onstage in New Zealand.

We haven't seen the Biebs let loose like this in a while -- since cleaning up his act -- but during the duo's gig, Biebs says he took a dare and chugged from a bottle of Hennessy.

Bieber never totally laid off booze, even though he battled some serious drug issues, but he's gotta be having some regrets today ... or at least a headache.