Kathryn Dennis will have to do some jail time in connection to her DUI arrest last year ... 'cause a judge sentenced her to serve 30 days.

According to online booking records, Dennis was taken into custody aropund 7 PM ET on Friday night in Berkeley County, South Carolina to begin her 30-day incarceration.

We broke the story ... Dennis was arrested last year after cops said she got into a three-car collision in Goose Creek, South Carolina.

Documents obtained by TMZ included a report in which an officer said Dennis appeared impaired with "glossy eyes and an odor of alcohol coming from her person."

Dennis has dealt with substance abuse issues over the years ... famously going to rehab in 2016 after losing custody of her children.

Kathryn broke down during her arrest ... crying and threatening the police officer who arrested her in the midst of a total meltdown caught on video.

Dennis was on "Southern Charm" for eight seasons ... leaving the show back in 2023. She shares two children with Thomas Ravenel.