Kanye made a deal to buy a mega-unit in a super, super exclusive Miami Beach condo building, dubbed "Billionaire Beach Bunker" ... TMZ has confirmed.

The 18-story building is occupied by hedge fund gazillionaires and ridiculously rich land developers.

We're told Kanye scoped the place out on his own while in town for Art Basel, and Kim had never even seen it. Kanye has fairly deep ties to the city. He's a regular at the Basel. Kim also did a show there ... "Kourtney & Kim Take Miami."

Our sources say Kanye secretly made the offer this month and surprised Kim on Xmas day.

The unit sounds awesome. It's 4,700 square feet with 4 bedrooms, 5 1/2 baths and an insanely large terrace. The building is tricked out with a gym, pool and spa.

Obviously, it's right on the beach.

They actually got a deal ... sort of. It was originally listed at $15.5 million.

Page Six first confirmed the story. The deal is set to close next month.