Mariah Carey gave an impromptu performance at the swanky Carlyle on the Upper East Side Tuesday — so she was a surprise singer, though she’s skirted rumors she’ll appear on “The Masked Singer.”

We’re told that Carey — dressed in a floor-length red sequinned dress — was with two pals and a “large bodyguard” at the hotel’s venerable Bemelmans Bar, when she spontaneously decided to get up and sing with the pianist.

A gobsmacked witness told us, “I was saying, ‘I think that’s Mariah Carey’ to a friend.” When the diva overheard the guy, she replied, “It’s because I am Mariah Carey.’ ” The audience member added, “She was very nice.”

Carey is a fan of the famed uptown spot: She posted a picture of herself and some friends there online in 2012, with the comment, “During an impromptu visit to the Carlyle, singing jazz standards I’ve loved since childhood.”

Carey has recently deflected rumors that she’ll appear on “The Masked Singer,” coyly telling “Entertainment Tonight” when asked if she will be on the show, “Bye, girl, bye.” She’s teased an upcoming memoir and will celebrate the 25th anniversary of her “Merry Christmas” album with a re-release and an accompanying holiday tour.