Abstract: An underwater pile driver assembly that includes a vertically disposed tubular housing that may slidably engage the upper end portion of a pile, with the housing having an upper open end, and the housing supporting an anvil that may be placed in abutting contact with the pile. An impact imparting ram is slidably and sealingly mounted in the housing, and may be successively raised to a first position therein. Pump means are provided to successively evacuate water from the interior of the housing between the ram and anvil. After each evacuation the ram is released to be forced downwardly by the hydrostatic head imposed on the upper surface thereof to impact the anvil, with the force imparted to the anvil being transverse to the pile to drive the latter downwardly. The greater the distance of the upper end of the housing from the surface of the water, the greater will be the magnitude of the downward force exerted on the pile each time the ram impacts the anvil.