The singer, who will begin her 'Born This Way Ball' world tour in South Korea on April 27, insisted that she will take the cup around the globe with her to every show, as she is a true fan of the 'Toxic' star.
"Real fans never turn their back on you #WillAlwaysLoveBritney Some stars really are as sweet as you thought they were," GaGa tweeted, along with a picture of her Britney thermos.
Spears replied: "Wonder what @LadyGaga keeps in that thermos??? Cant WAIT to see you on tour. MISS YOU !!!"
GaGa was spotted attending one of Spears's 'Femme Fatale' tour shows last August.
Meanwhile, the 'Judas' performer recently held a meeting about the use and positioning of fog in her 'Born This Way Ball' tour stage show.
Tickets for Lady GaGa's 'Born This Way Ball' UK tour are now available here.
Watch the music video for Lady GaGa's 'Marry The Night' below: