An automatic clamping device is disclosed for holding planar workpieces such as: doors, windows, panels, plasterboard, etc. The planar workpiece is held in an upright position by clamping bars actuated by the weight of the workpiece. Automatic clamping is accomplished by placing the workpiece through a slot at the top of the clamping device and lowering it onto a flexible strap connected to movable bars which, under the weight of the workpiece, move laterally towards and clamp the workpiece. The workpiece is released by simply lifting it up through the slot and an elastic member will automatically return the bars to an open position.