EXCLUSIVE

"Teen Mom" star Amber Portwood has been sentenced to five long years in the big house ... again -- after asking an Indiana judge to reimpose her sentence, stemming from a December drug arrest.

Portwood was in court this morning when the judge reimposed the sentence. Her attorney asked for leniency, but the judge was unmoved ... insisting he had no choice but to enforce the terms of Amber's plea agreement.

Portwood's five-year prison sentence was previously stayed on the condition she complete a drug rehabilitation program -- but as we first reported, Portwood gave up on the program ... and asked to go to prison instead.

The judge said he would refer Amber to the Indiana Department of Corrections therapeutic community program -- an incarceration-based treatment program.