The pop star posted a photo with BSB's A.J. McLean on Instagram late Friday after McLean took in Spears' "Piece of Me" show in Las Vegas.
"Huge confession... biggest @backstreetboys fan! Tonight was truly a dream for me," Spears captioned the picture.
Also in attendance was McLean's bandmate Kevin Richardson, who posed for a photo with McLean at the show.
"Britney bitch!!!" McLean captioned the nostalgia-inspiring selfie.
The Backstreet Boys made headlines earlier this week when McLean and Nick Carter joined Gigi Hadid onstage for her Lip Sync Battle performance of their hit song "Larger Than Life."