Mariah Carey will most certainly get the best room in the joint when she performs at the grand opening of the newest casino in Macau ... because her billionaire boyfriend owns it. 

Studio City -- that's the name -- promises to be Asia's answer to Vegas. We're told Mariah's getting a check in the high 6 digits for the one-off gig, set for Oct. 27th.

BF James Packer is an investor, but the booking is not rank nepotism. The owners are banking on Mariah being enough of a draw to bring Chinese bigwigs to the casino's tables.  

Times have been tough in Macau because of the free falling Chinese economy.

We're told Packer's got other celebs filling the front row, including pals Brett Ratner, Martin Scorsese, Robert De Niro and Leonardo DiCaprio.

Added bonus: Studio City one ups Sin City with a high tech Ferris wheel built into the casino's hotel.