Dekker, 21, is best known for playing hero John Connor on the recently canceled TV series "Terminator: The Sarah Connor Chronicles." He has also appeared in the films "From Within" and "My Sister's Keeper."
TMZ said he was arrested after he allegedly struck a bicyclist with his car. He reportedly was charged with suspicion of DUI and causing bodily harm.
His bail has been set at $100,000, the entertainment news Web site said.
TMZ did not identify the bicyclist or say how badly he or she was hurt.