Floyd Mayweather is already spending his new fortune -- dropping millions on a brand new red Bugatti, TMZ Sports has learned.

We spoke with Floyd's car guy, Obi Okeke -- aka Doctor Bugatti -- who tells us Floyd hit him up 2 days ago to purchase a "triple red" 2012 Bugatti Grand Sport Convertible. 

Obi says the car is "one of one" -- and costs $3.3 MILLION!!

We're told Floyd wanted the car to make a splash on his way to the weigh in for the McGregor fight Friday afternoon. 

Floyd's expecting to make around $300 MIL for the fight ... so, what's a couple mil on a new whip, right?