Nostalgia just slammed into Vegas … ’cause when New Kids On The Block kicked off their Right Stuff residency, members of a rival boyband crashed the stage — and the crowd lost it!

Catch the clip — NKOTB were hyping up the crowd last weekend at the Park MGM, asking who wanted to marry them back in the ’90s … and while fans screamed, Backstreet Boys’ Howie Dorough popped up with a cheeky banner that read, “I Do.”

Howie later pulled up with AJ to crash the stage … and it was nothing but boy band bromance — hugs, laughs, and major 1990s nostalgia as they embraced NKOTB like long-lost brothers.

If you were expecting a boy band showdown — think again. Even with BBs launching their Millennium Sphere residency later this month, both groups proved there’s more than enough room (and love) to go around on the Vegas Strip.

But with NKOTB already kicking things off with a bang, you can bet it’s lit a fire under the Backstreet Boys — and sources tell us they’re grinding hard, rehearsing 12 hours a day, 5 to 6 days a week, to gear up for their massively anticipated Sphere takeover.