Floyd Mayweather -- Baby Mama Sues for $20 Mil ... He's a Despicable Liar

Floyd Mayweather could lose tens of millions of dollars from his big payday if his baby mama gets her way ... because she's just filed a lawsuit claiming he ruined her with lies to save his own ass.

Josie Harris, who has 3 kids with Floyd, claims he lied through his teeth in an interview with Katie Couric just 2 weeks before the big fight ... when he claimed Josie was in a drug-fueled rage and he had to "restrain" her during their infamous 2010 domestic violence incident.

Point of fact ... Floyd was convicted of domestic violence and spent two months in jail.

Josie recounts her terror in the lawsuit, explaining how she and Floyd had broken up ... but he flew into a jealous rage that night, broke into her home and viciously attacked her while she was sleeping on her couch ... and her kids saw part of the beating.

Harris says she is now labeled a drug addict thanks to Mayweather's lies -- and was embarrassed and humiliated on a global scale.

Her lawyer, Dan Friedlander, writes in the lawsuit -- Floyd's lies to Couric "were knowingly fabricated in order to hype Mayweather's upcoming fight with Pacquiao, to avoid bad press by deflecting responsibility for beating the mother of his children."