Abstract: A truck trailer door safety door stop is provided for preventing a truck trailer door from accidentally falling closed. The door stop may be retrofitted onto a conventional truck trailer door of the roll up type including a plurality of hinged panels which ride along rails by means of roller wheels. The door stop includes a glide member for guiding a bottom panel of the roll up door along the rail without engagement therewith. When the door is in a fully opened position, the disengaged bottom panel is pulled out of alignment with the rail and separated therefrom by means of a spring. In response to a sudden forward movement of the truck, which would normally cause such a door to accidentally close, the door will be forced directly rearward and be prevented from falling down the rail. A guide bar may be provided for supporting the bottom panel in a positive lock open position.