Archimedes's screw (also the Archimedean screw) is one of several inventions and discoveries reputed to have been made by Archimedes.

The Archimedes's screw is a simple machine used primarily to transfer water from a lower-lying body of water into irrigation ditches. Another interesting application is "stealing" land from under sea level in the Netherlands. Since the primary objective in this case is to lift water to a given height rather than simply move it from a river to the irrigation field, more than one machine was used to successively lift the same water volume, due to the limitations of this machine.

The machine consists of a screw inside a hollow pipe. The lower end of the device is put in the water, and the screw is then turned (usually by a windmill or by animal or human labor). As the bottom end of the tube turns, it scoops up an amount of water. This puddle of water will slide up in the spiral tube as the shaft is turned, until finally it falls out from the top of the spiral tube and feeds the irrigation system.

The interface between the screw and the pipe does not need to be perfectly water-tight because of the relatively large amount of water being scooped at each turn in respect to the angular speed of the screw. Also, water leaking from the top section of the screw leaks in the previous one and so on, so a sort of equilibrium is achieved while using the machine, thus reducing the decrease in efficiency.

Modified Archimedean Screws are also used for vehicle propulsion. This works best over water, snow, sand, and other soft surfaces, although it can work as effectively over solid surfaces. This is normally done with pairs of screws: If they are rotated in opposite directions the vehicle will move forward; the same direction the vehicle will move sideways; different speeds cause the vehicle to turn. To date no production vehicles have been created using the screws.

See also: progressive cavity pump, helical pump