While on stage in Atlantic City, the 29-year-old singer revealed that Gaga, 25, real name Stefani Germanotta, was among the audience, according to E! Online.
Britney told the crowd: 'I have a very special guest tonight... Lady Gaga is here.'
Spears later tweeted about the gig, thanking the You and I singer for supporting her.
She wrote: 'So amazing to have the beautiful @LadyGaga at the show in AC tonight. Love you girl.'
The Judas hitmaker was said to have enjoyed the concert, especially Britney's performance of I Wanna Go.