Canelo Alvarez went straight from whaling on Triple G's face to belting out tunes ... hitting a nightclub in Vegas after his big win Saturday -- and serenading his fans for several minutes on stage!!
The 32-year-old star boxer grabbed the mic at Zouk Nightclub at his post-fight celebration ... and he sang his face off throughout the evening after beating Gennady Golovkin by decision.
Video shot from bystanders in the club's crowd shows Canelo sang at least two songs -- hitting all kinds of notes in Spanish as everyone in attendance cheered him on.