'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 ... and we're told it was a star-studded crowd with some of the biggest names in wrestling and music.
The wedding looked awesome, but the reception turned into a freakin' rock concert ... with O-Town, Yellowcard, Bowling for Soup and We the Kings playing multiple songs for the crowd.
Of course, it's pretty rare to see 'NSYNC performing nowadays -- especially with Justin Timberlake andJC Chasez distancing themselves over the years.
The quintet reunited in 2013 to perform at the MTV VMAs ... and a JT-less 'NSYNC joined Ariana Grande at Coachella in 2019.
While Fatone has recently said "never say never" to the idea of a reunion ... we'll have to settle for this awesome performance for now.