A rotor ship is a type of ship designed to use the Magnus effect for propulsion. The ship is propelled, at least in part, by large powered vertical rotors, sometimes known as rotor sails. German engineer Anton Flettner was the first to build a ship that attempted to tap this force for propulsion, and ships using his type of rotor are sometimes known as Flettner ships.

The Magnus effect is an observable phenomenon commonly associated with a spinning object moving through a fluid. A lift force acts on the spinning object. The path of the object may be deflected in a manner not present when the object is not spinning. The deflection can be explained by the difference in pressure of the fluid on opposite sides of the spinning object. The strength of the Magnus effect is dependent on the speed of rotation of the object.