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.

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