'Teen Mom' Star Amber Portwood Checks In to Prison

So it begins -- TMZ has learned "Teen Mom" star Amber Portwood officially checked in to an Indiana prison this morning to serve out a 5-year sentence stemming from her December drug arrest.

Law enforcement tells TMZ, 22-year-old Amber was transported from a local jail to the Rockville Correctional Facility at approximately 7:45 AM.

TMZ broke the story ... Amber was sentenced to 5 years in prison for drug possession back in January -- but the judge offered to keep her out of the big house if she completed a drug program.

Amber took the offer, but eventually quit the program -- deciding to serve time in prison instead.