MTV star Britni Thornton was arrested for a DUI, but based on her mug shot ... she ain't even mad.

"The Challenge" contestant was busted in Georgia Saturday night around 10 PM and taken to the Columbia County jail, where she was booked for driving under the influence of alcohol/drugs and failure to maintain lane ... according to the report.

Britni's all smiles in her mug shot, though ... and was released after posting a $1,100 bond.

Thornton competed on the recent season of "The Challenge: Vendettas" and will reportedly be on the upcoming season of "The Challenge: Final Reckoning."