'NSYNC is BACK -- well, partially -- the boy band hit the stage after Alexa Bliss and Ryan Cabrera's wedding over the weekend ... and the WWE Superstar joined them on stage!!
TMZ Sports has obtained video of the epic moment (thank you, Mojo) ... showing the wrestler singing and dancing her heart out alongside Lance Bass, Chris Kirkpatrick, and Joey Fatone as they perform the 2000 hit, "Bye, Bye, Bye."
Alexa and Ryan got hitched in Palm Springs on Saturday .