Jay Cutler is back behind bars … TMZ Sports has learned he reported to a Tennessee jail on Monday to begin serving his DUI sentence.

Records we’ve obtained show the ex-NFL star was booked into Williamson County Jail at around 8 AM PT … and the former quarterback is now expected to remain there until Friday.

Cutler, of course, was ordered to spend four days in the slammer last month … after he pleaded guilty to one count of misdemeanor DUI.

A judge also required Kristin Cavallari‘s ex-husband to pay a $350 fine, and go on supervised probation for one year. Cutler additionally was ordered to attend a DUI safety class … and his Tennessee license was revoked for one year.

It all, as you know, stems from Oct. 17, 2024 … when the 42-year-old was accused of exhibiting signs of being intoxicated following a traffic accident in Franklin.

Authorities alleged Cutler rear-ended a car, and when officers showed up to investigate the scene, he reeked of booze, had bloodshot eyes … and was slurring his words.

Cutler — who was arrested over the incident — was later sued by a man who claimed Cutler was “at fault” for the accident. In the lawsuit, the plaintiff, Perry Lee, stated he sustained injuries in the collision and is in daily pain.