Mariah Carey Signs Rihanna's Breast During Christmas Concert in NYC: Video

Mariah Carey didn't let the flu get in the way of her performing in NYC Tuesday evening — or delivering a personalized gift to none other than Rihanna.

Mariah wrapped up her Christmas Time tour at Barclays Center with last night's show — after canceling a few concerts earlier this month while battling the flu.

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The Barclays show was extra special because Rihanna popped up in the packed crowd to watch Mariah do what she does best: sing!

At some point, Mariah spotted Rihanna and came down from the stage to greet her.

But that wasn't all ... check out video posted by Jason Lee on IG ... Mariah, microphone in hand, strolls up to Rihanna, who suddenly requests a sharpie from a man standing nearby.

The man produces a red marker and hands it to Mariah as Rihanna asks Mariah to sign her breast.

Mariah most certainly obliges as Rihanna grabs the mic from her hand and announces, "Mariah Carey is signing my tit, y'all."

The two stars then hug — and the rest is history!