Jay Cutler Reports To Jail For DUI Sentence

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.

Launch Gallery Launch Gallery GettyJay Cutler On the FieldCutler, 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…