Backstreet Boys' Howie Gives Surprise 'I Want It That Way' Performance in Vegas

Backstreet Boys fans who couldn't get tickets to the band's residency at the Sphere shouldn't be too bummed ... as BSB member Howie Dorough just gave an impromptu performance while out and about in Las Vegas.

Check it out ... the boy bander hit the stage at Barbershop Cuts & Cocktails at The Cosmopolitan in Sin City Thursday night, where he was celebrating his 52nd birthday with his wife, Leigh Boniello, and friends.

While the singer wasn't slated to perform, given he was the guest of honor, HD surprised fans with a solo performance of Backstreet Boys' hit, "I Want It That Way."

In the vid, Howie belts out the popular '90s ballad, while being backed up by the house band, The Regulators, and a crowd of adoring fans. He later took a moment to mingle with fans, taking a number of selfies with those treated to the surprise show.

Howie is currently in Vegas for BSB's "Into the Millennium" residency ... which just announced an extension to its already wildly successful run.

It's no wonder Howie was in an extra celebratory mood this week.

Still, the Barbershop is known for having its fair share of surprise performances ... with the likes of Bruno Mars, Sir Rod Stewart, Sammy Hagar, among others hitting the intimate venue's stage over the years. Even Flavor Flav was enjoying the performance.