The collaboration will feature on the star's upcoming Christmas album Under The Mistletoe, which is released on November 1.
Teasing fans on Twitter, Bieber asked: "hmmmm....u really wanna know huh....maybe it is time i tell you."
He then posted a video which featured Carey on a roof top. She says: "What's up? I'm Mariah Carey. I am here today to tell you a little secret.
"Justin and I are doing this duet together and it's my song 'All I Want For Christmas Is You', which I wrote and produced."
She added: "I'm very excited about it and I think a lot of you are very surprised by this and you're going to be even more surprised when you hear it because the song sounds great."
Bieber recently revealed that his new festive LP will also feature collaborations with Boyz II Men and his mentor Usher.
Watch Mariah Carey announce her duet with Justin Bieber below: