Abstract: A system for automatically replacing ballast water in a ballast tank of a ship comprises a water inlet port on a part of the ship where the water pressure acting on the ship when in forward motion in the water is higher than the pressure of the ballast water being replaced, a water discharge outlet in the ballast tank and a water inlet passage extending between the water inlet port and the ballast tank for introducing water into the ballast tank at a higher pressure than the ballast water in the tank. In this way the ballast water is forced from the ballast tank through the discharge outlet to be replaced by water introduced through the inlet port. A method for replacing ballast water in the ballast tank of a ship is also provided.