DeBarge Brother -- Mark DeBarge DeBUSTED ... For Drugs

Mark DeBarge (from DeBarge) was arrested for drug possession this week in a Hollywood hotel ... TMZ has learned.

Law enforcement sources tell us, Mark was in his hotel room with some woman early Wednesday morning -- when the woman's parole officer stopped by for a routine check-up.

We're told the officer searched the room and discovered narcotics -- it's unclear exactly what.

Once the drugs were found, we're told Mark and his female companion were placed under arrest. Mark was booked for narcotics possession and has since been released.