The "To Love Somebody" singer sat down on the stage, told the crowd, "don't be terrible judges," encouraging them to support her, then announced Bellucci would be singing the classic Etta James song, "At Last" and handed her his microphone.
Bellucci, showing no hesitation, belted out a rendition that seemed to stun the "Haven't Met You Yet" singer. Her sister Allison Bellucci shared the video on Twitter:
Bellucci's performance was received with cheers from the crowd. Buble himself could be seen saying "Wow" and clapping when she hit the high notes.
"That was so beautiful," Buble told Bellucci after she was finished. "That may not be your last time here. That was incredible."
"Too incredible if you ask me," the singer added, jokingly.
Bellucci, a junior at the University of Delaware from West Hartford, Connecticut, told ABC News she wants to teach music to children and record her own.