Starr was arrested and booked on suspicion of felony possession of a controlled substance early Thursday ... according to Salt Lake City PD arrest documents.
Starr was a patient on the third season of "Celebrity Rehab" back in 2009. He also appeared on the most recent season as an example of a celeb who got clean after appearing on the show.
It's unclear at this point what drug -- or drugs -- Star allegedly had at the time of his arrest. He is still behind bars ... held on a $20,000 bond.