The song will feature on the 17-year-old's highly anticipated Christmas album, Under the Mistletoe, which goes on sale on November 1.
Along with Justin's announcement was a video from Mariah, 41, in which she said the pair have recorded All I Want For Christmas Is You together - Carey's massive Christmas hit.
Mariah broke the news in a video posted on Twitter, saying: 'Justin and I are doing this duet together. It is my song All I Want for Christmas Is You, which I wrote and produced.
'I think a lot of you are already very surprised by this and you're going to be even more surprised when you hear it. The song sounds great. I'm very excited for everybody to hear it this Christmas and to be totally festive with our festive collaboration.'