The pair performed a rendition of the 77-year-old music legend's disco anthem 'No More Tears (Enough Is Enough)', which she recorded with Donna Summer in 1999, at Barbra's concert at Chicago's United Center.
The secret appearance had been teased by the 'Woman in Love' hitmaker the day before, when she posted a picture of the pair getting a manicure together.
She simply captioned the post: ''Made a new friend.''
And 'No Tears Left to Cry' hitmaker Ariana posted a black heart emoji underneath the snap.
After the show, Barbra shared a selfie with the pop superstar - who was clutching a water bottle - with the song sheet in her hand.
She wrote: ''Secret's out. Thank you @arianagrande.''
And once again in the comments under the Instagram post, Ariana gushed about how their performance together was ''the most special and cherished experience ever''.
She wrote: ''i'm sobbing love you so much. i cant. get over this. it's just.... the most special and cherished experience ever. thank you. (sic)''
'Meet The Fockers' star Barbra has been surprising fans with surprise duets all summer.
During her headline set at London's Hyde Park last month, the Hollywood legend was joined by Kris Kristofferson for the first ever live performance of their 'A Star Is Born' duet 'Lost Inside of You', over four decades after she and the 'Me and Bobby McGee' singer, 83, starred in the 1976 version of the musical drama, which was adapted last year with Lady Gaga and Bradley Cooper in the leading roles.
If that wasn't enough of a treat for fans, Barbra - who drew in a star-studded crowd including A-listers Antonio Banderas, Richard E. Grant and Ralph Fiennes - brought out special guest Lionel Richie for the live debut of 'The Way We Were', which they recorded for her 2014 duets LP 'Partners'.
The 'All Night Long' hitmaker - who had performed with Stevie Wonder the night before at British Summer Time Hyde Park - admitted it was a mission to sneak into the festival and keep the secret appearance under wraps and that he was ticking off one his biggest dreams by getting to perform with Barbra.
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