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 GettyCutler, of course, was ordered to spend four days in the slammer last month .