AMBER PORTWOOD Strikes Plea Deal in Drug Case 5 Years in Prison, BUT ...

"Teen Mom" star Amber Portwood has struck a plea deal after her drug arrest last month -- requiring her to serve a 5-year prison sentence -- but TMZ has learned, there's still a way for her to keep her freedom.

According to the deal, law enforcement sources tell TMZ, Amber can stay out of the big house ... provided she completes a court-mandated drug rehabilitation program.

As part of the deal, Amber pled guilty to one count of possession of a controlled substance and one count of probation violation.

The entire case will be dismissed if Portwood successfully graduates from the Madison County Drug Court program -- but we're told, if she screws up ... she'll be tossed in the pokey immediately, no questions asked.

A hearing is set for February 6th -- when the judge will decide whether to approve the deal.

TMZ broke the story -- Amber was arrested on December 19 for drug possession and failing to provide a urine sample (a stipulation of her probation). She is still in custody.