Gaga, 27, attended Britney's 'Piece of Me' show at the Planet Hollywood Resort and Casino and hung out with the singer backstage.
The two popstars posed for a photo in Britney's dressing room as Gaga declared that they were enjoying a "Monster party"!
Captioning the snap, Gaga said: "Me and Brit Brit <3 #Applause4Britney @britneyspears we had so much fun at your show!"
'Toxic' singer Britney was all smiles as she cuddled up to Gaga in a sheer glittered bodysuit. We'd love to see these two duet!
'Bad Romance' hitmaker Gaga also had plenty of praise for Britney's dad, Jamie Spears, after he cooked her food. She tweeted: "And Britney's dad gave us hommeade BBQ, it was one of the best things I've ever eaten in my LIFE."
Gaga, who headed to the show wearing a navy blue mini dress, later added on her Little Monsters site: "She [Britney] looked so gorgeous, the show was so much fun, and her dad gave us THE BEST BBQ. It was a #MonsterParty!"
Britney kicked off her two-year Las Vegas residency on 27 December and her show, which features more than 20 of her greatest hits, will run for 50 dates across a two-year period.
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Watch Britney's video for 'Work B**ch' below: