12:23 PM PT -- Berkeley County Detention Center just released Kathryn Dennis' mug shot.

Kathryn Dennis is facing some serious legal trouble after getting busted Monday night for driving under the influence in South Carolina ... TMZ has learned.

Cops in Goose Creek, SC booked the former "Southern Charm" star after they say she was involved in a 3-car collision. In documents obtained by TMZ, a responding officer described Kathryn's alleged state at the accident scene as appearing impaired with "glossy eyes and an odor of alcohol coming from her person."

This is her first offense with drinking and driving ... though it isn't her first run-in with the law.

Remember, Dennis was identified as a suspect in an elementary school hit-and-run in November 2023 -- she was never arrested, however.

As for Monday's events, KD was also booked for driving with an open container.

She's also had substance abuse struggles in the past. Kathryn lost custody of her children with ex/costar Thomas Ravenel in 2016, after testing positive for marijuana and entering rehab.

The couple's gone back and forth in their custody battle, with Thomas reportedly gaining full custody last year.

Kathryn and Thomas -- who have a 29-year age difference -- first linked up on their Bravo show ... embarking on a tumultuous on-and-off relationship for several years. They welcomed their children, Kensington and Saint, in 2014 and 2015, respectively.

The couple split for good in 2016.

Kathryn announced her exit from "Southern Charm" in 2023 after 8 seasons on the show ... noting she was looking forward to what was next. Things seemed to be going well for Kathryn ... who announced in March she was heading back to school.

Unfortunately, things have taken a 180 turn since that positive update.