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. The Barbershop Cuts & Cocktails

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 ming

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