Madonna has some loyal followers, and one of them is Katy Perry! The Queen of Pop's fans were treated to a surprise appearance by Perry in Inglewood, Calif. as a part of Madonna's Rebel Heart tour. Things got a little X-rated during the show after Perry grinded on Madonna onstage.

Perry came up during Madonna's song "Unapologetic B---h." Every night of the tour, the "Like a Virgin" singer named someone from the crowd an "unapologetic b---h," but this time, it was Perry's turn, according to The Huffington Post. The "Firework" singer walked out in front of the crowd and dropped it low right on Madonna before the 57-year-old returned the favor and spanked Perry on her backside.

"I love you, Mom!" Perry said to Madonna after the dirty dancing.

This was not the first time that Perry and Madonna have collaborated together. Perry appeared as a guest curator for Madge's Art for Freedom project in 2014. A couple of months later, the two were on the cover of V Magazine together, and they walked the red carpet at the Met Ball with matching Moshino dresses earlier this year, according to MTV.

Perry watched the rest of Madonna's show from the audience along with Salma Hayek, Kathy Griffin, Mila Kunis, Ashton Kutcher, Madonna's ex-boyfriend Tony Ward and Jay Z and Beyoncé.

Check out the video of Perry and Madonna below.