The "Bad Romance" hitmaker and Ariana's collaboration enters Britain's Official Charts Company rundown at number one on the same day GaGa releases her new album, "Chromatica".
Speaking to OfficialCharts.com, the singer/actress reveals she's thrilled a song she wrote at an extremely low point is now a huge hit.
"If someone told me the day I wrote 'Rain on Me', a day I was crying water like misery, that this song would someday be number one, I never would have believed them," she explains. "Thank you for celebrating this song. It's my gift to you, to celebrate yourself. I hope the whole world is dancing."
"To all my U.K. monsters (her fans), I love you so much from the bottom of my heart. Thank you for making me number one, and I hope you feel number one every time you listen to this song."
GaGa and Ariana now each have six U.K. number one singles and their new hit becomes the first all-female collaboration to top the chart in six years, following the "7 Rings" hitmaker's "Bang Bang" collaboration with Jessie J and Nicki Minaj, which shot to number one in 2014.
Last week's number one, DaBaby and Roddy Ricch's "Rockstar" falls to two, S1mba and DTG's "Rover" is at three, Doja Cat's "Say So" slips two places to four, and Powfu's collaboration with Beabadoobee, "Death Bed", completes the top five.
In the albums chart, The 1975 win a close battle with YouTuber-turned-rapper KSI to top the rundown with their new record "Notes on a Conditional Form". KSI's "Dissimulation" is at two, ahead of Lewis Capaldi's "Divinely Uninspired to a Hellish Extent".