Floyd Mayweather Jr. Surrenders for Jail Stint

Floyd Mayweather Jr. has officially begun his 90-day jail sentence for domestic violence.

Floyd turned himself in to a Vegas jail moments ago -- where he'll serve the remaining 87 days of his sentence. Floyd was given a 3-day credit for time served after his arrest. 50 Cent was present in court for support.

The judge praised the boxing legend for having already completed 45 hours of community service -- 20 hours at Habitat for Humanity, 10 hours at a Nevada food bank called Three Square, and 15 hours at Las Vegas Rescue Mission. Floyd has 55 hours left to complete.

In addition, Floyd has already completed 21 of his 54 required domestic violence classes.

Floyd was supposed to surrender back in January, but the judge postponed his jail date so he could fight Miguel Cotto on May 5th. It was worth it ... Floyd won ... and the fight raked in $94 MILLION in pay-per-view sales. For himself, Floyd earned a reported $32 million ... at least.

Floyd pled guilty to misdemeanor domestic violence back in December for beating up his baby mama in front of their kids.