A method and apparatus for washing sheets of plastic material originating from industrial and agricultural waste, for subsequent reuse thereof as a raw material. The sheets of plastic material, precut into soiled pieces of large dimensions, and a water flow are supplied to a washing tank containing a water bath; a screw stirrer generates intense stirring of the water bath and recirculation, inside the tank, of the soiled plastic pieces that causes the contaminating material to be detached and separated by sedimentation from the plastic pieces. The cleaned pieces of plastic material of large dimensions, devoid of contaminant, which float in the water bath are removed by dragging, in a floating condition, along a discharging channel by the combined action of a dragging device and of a water flow that exits the washing tank; the dragging device, consisting of an endless belt, extends from one side of the washing tank along the discharging channel, supported in a freely tiltable manner.