Floyd Mayweather -- 90 Days in Jail for Beating Up Baby Mama

Floyd Mayweather Jr. got hammered by a judge, who just sentenced him to serve 90 days in jail for beating up his baby mama in front of their kids.

Floyd was sentenced to 6 months in jail, but 3 months of that sentence was suspended. He also gets 3 days credit for time he has already served.

TMZ broke the story ... the boxer was arrested in September 2010 -- after his ex-girlfriend accused him of striking her "multiple times in the head with his fist" and then threatening to kill her.

Floyd struck a plea deal with prosecutors in the case -- in which he pled guilty to misdemeanor domestic violence ... in exchange for getting a pass on the felony battery charge.

Floyd also pled no contest to two misdemeanor harassment charges -- after allegedly threatening to beat his kids too during the incident.

The judge was especially pissed that Mayweather struck his baby mama in front of their kids.

The judge also ordered Mayweather to complete 100 hours of community service and a 12-month domestic violence program. He was fined $2,500 ... which to Floyd is like a poker hand.

The boxer must turn himself in on January 6, to begin serving his sentence.