The star joined Yoko Ono and her Plastic Ono Band on stage in Los Angeles to celebrate the Beatles singer, who would have been 70 this year.
New York Daily News reports that Ono praised GaGa, who wore a see-through outfit which exposed her rear.
She is quoted as saying that the 'Poker Face' singer was "so brilliant and such an inspiration to so many women".
Also performing at the event were Lennon's son Sean, rapper RZA and Sonic Youth members Thurston Moore and Kim Gordon.
GaGa later tweeted: "WE ARE PLASTIC ONO. I got to sit in as a guest musician tonight, what a legendary band+mother, Yoko (sic)."
Further tribute concerts will take place in New York, Iceland and Liverpool.