Katt Williams -- Loses Custody of Daughter

Katt Williams has lost custody of one of his children ... TMZ has learned.

According to court docs obtained by TMZ ... Crystal McGhee just won custody of their 8-year-old adopted daughter, Leanne. It's unclear if Katt and Crystal ever married.

What is clear ... the judge awarded sole legal and physical custody to Crystal. The judge gave Katt visitation rights -- alternate weekends.

The judge also awarded Crystal visitation with 2 of Leanne's siblings. It's unclear if Katt has custody of Leanne's siblings but we do know he has 7 adopted kids.

No comment from Katt.