Mischa Barton -- Repo Man Coming For Her Audi Q3

Mischa Barton better put her brand new Audi Q3 in high gear if she doesn't want the repo man to take it back ... because she hasn't paid a dime on her lease in 4 months.

Mischa signed the car lease at the end of March. She put down $2,300 and agreed to $696.29 a month for 3 years ... but she's only made one payment.

According to legal docs, the leasing company got impatient and demanded she return the Audi, but she didn't. So now the leasing company is turning up the heat, demanding full payment -- $40k.

Barton's financial problems are well-documented ... she sued her mom Nuala back in April, claiming the momager pocketed a big chunk of her earnings and even forced her out of her own house.