Leighton Meester just won her legal war against her mom -- after the "Gossip Girl" star sued her last year ... claiming the woman diverted money Leighton had sent to her little brother ... and she spent it on things like plastic surgery, Botox, and hair extensions.
TMZ broke the story -- Leighton filed the lawsuit claiming her little brother has serious medical problems ... so she sent home $7,500 a month to help cover his expenses.
Leighton's mom, Constance Meester, tried convincing the judge Leighton had promised to take care of her and that's why she spent the cash. But the judge pooh-poohed mom's claims, ruling there was no such agreement.
Leighton actually didn't sue to recover any money from her mom -- she just wanted the judge to make her mother go away.