A tonneau cover is provided that is attached at its most forward end to an adjustable tensioning rail which spans the front end of a pickup truck box. The tensioning rail is adjustably attached at its outside edges to two side rails which are attached to the inside of a pickup box. The adjustable attachment of the tensioning rail to the side rails is accomplished by the use of a tensioning screw and two attachment blocks. As the tensioning screw is moved forwards and backwards by the use of the adjustment knob, the tensioning rail is moved in a corresponding manner. This adjustment is preformed when the tonneau cover is in the open position, and thus, allows a user to obtain the desired amount if tightness when the tonneau cover is subsequently placed in the closed position.