Thomas Dekker -- the guy who played "John Connor" on the Terminator TV show -- was arrested last night on suspicion of DUI after allegedly hitting a bicyclist.

Law enforcement sources tell TMZ the 21-year-old "The Sarah Connor Chronicles" actor was busted around 9pm Wednesday in the San Fernando Valley.

On his booking sheet, Dekker is listed as being 5'11", 155lbs.

He's currently being held on suspicion of felony DUI and causing bodily harm. His bail is set at $100,000.