Floyd Mayweather's just days away from his birthday ... and while he hasn't yet completed his 48th trip around the sun, the 50-0 boxing legend's kicking off the celebration a bit early!

TMZ Sports has learned Mayweather, along with ~400 of his closest family and friends, traveled to South Beach for a week of fun ... all to celebrate TBE's 48th birthday, officially on Monday.

We're told Floyd has spent a fortune on the parties ... dropping upwards of $5 million! Mayweather is putting his guests up at several fancy hotels throughout Miami Beach.

But, it won't just be 7 days of drinking (though, there will be some of that) ... Mayweather has a ton of different events planned.

There will be a beach party, an ATV tour, roller-skating (one of Floyd's favorite activities), bowling, and more.

Mayweather will also throw an event at a local mansion ... as well as on a yacht.

As for who will attend, the guest list isn't known, but you can bet many of Floyd's celebrity friends will be pulling up. It's also expected that there will be special musical performances.

And, if last year's any indication, Mayweather can expect extravagant gifts (he got a $150K timepiece in 2024).

The partying starts Monday night ... and will continue through February 24!