A hydraulic device for pumping molten metal and/or controlling its flow, comprising an enclosure that forms internally a cylindrical chamber, for the connection of at least one first duct and one second duct for the passage of molten metal, within which a piston can slide hermetically in order to pump and/or control flow, an insulating cushion of inert gas being interposed between the piston and the molten metal, the piston being associated with an actuator for moving it with a reciprocating rectilinear motion.