Justin Bieber Crashes Mariah Carey/French Montana Sesh

Justin Bieber made a very spontaneous decision to appear in a Mariah Carey and French Montana song.

We're told Mariah and French were working on a new song Thursday night at a Hollywood recording studio when Bieber just showed up.

Biebs confessed to Mariah he's a huge fan, so she played a portion of the song she and French were recording.

Bieber was so into it, he jumped into a booth and laid down some vocals that'll be featured in the song.