Katt Williams Wins Back Custody of Daughter -- Nanny Tried to Steal M

Katt Williams just won back custody of his 8-year-old adopted daughter -- after the girl's nanny allegedly lied to a judge to take her away from him ... this according to new legal docs.

TMZ broke the story last month -- a woman named Crystal McGhee went to L.A. court, seeking sole custody of Katt's daughter ... claiming she was the child's mother.

Katt failed to show up to fight McGhee's claim -- so the judge granted her request by default ... on the assumption McGhee was legit.

But Katt just filed new legal docs in L.A. County Superior Court -- claiming McGhee's nothing but a shameless fraud ... a former nanny "with no parental rights," who hasn't worked with the family for years. The judge has since overturned the previous order.

It's unclear what motivated McGhee to seek custody -- but Katt's legal docs suggest it has to do with California State parenting perks ... i.e. collecting government benefits.

Calls to McGhee were not returned.