There's Nothing Holdin' Me Back Shawn Mendes
Mother Monster showed up to support Brit on her 'Femme Fatale' tour.
Lady Gaga was a surprise guest in the audience at Britney Spears' gig in Atlantic City, US this weekend.
"I have a very special guest tonight," Brit told screaming fans.
"Lady Gaga is here!"
A spotlight then found Gaga in the crowd and she was beamed onto the big screens as Britney nodded a thank you in her direction for coming to see the show.
Gaga responded by holding her hands out and bowing twice for the 'Hold It Against Me' singer.
"So amazing to have the beautiful @LadyGaga at the show in AC tonight," Britney tweeted after the gig.
"Love you girl. -Britney"