The "Party Down South" star pled guilty to misdemeanor DUI and will serve 7 days at Davidson County Jail. According to court docs, obtained by TMZ, the judge hit Mattie with nearly a year behind bars, but suspended all but 7 days of her sentence.

Karla West, chief of staff at the Davidson County Sheriff's Office, tells TMZ ... Mattie will serve her jail time on weekends until she finishes her sentence. We're told it's normal procedure for similar DUI cases.

Once she's served 7 days behind bars, Mattie -- who's also been on MTV's "Floribama Shore" and "The Challenge" -- will be on supervised probation for close to a year, and will also lose her driver's license for a year. Third time for that too.

Mattie was arrested in August 2019 after a solo crash in Nashville. Cops say she told them she'd taken Hydrocodone and chugged 2 beers before getting behind the wheel.

Cops also say they smelled a strong odor of booze and that her speech was slurred. While getting treated at a hospital, she told cops she had 2 previous DUIs in Louisiana.

Pretty crappy trifecta to score.