Mariah Carey may have to get a restraining order against her billionaire fiance … to keep him in his seat during her concert.

James Packer joyously and (arguably) adorably celebrated Mariah’s return to her Vegas venue Tuesday night, busting a move that can only be called … unique.

He’s been her number one fan from the day they met … blasting her music from the speakers on his gigantic yacht, and following her from concert to concert.

James is pretty much the only one rocking out as Mariah owned the audience, but hey … who’s gonna tell a multi-billionaire to chill?