Abstract: A rotor sail is provided having a base (2) and a rotary cylinder (3) mounted on the base (2) in a manner permitting rotation about its longitudinal axis designed as the rotor axis (r), and a drive (5) for rotating the rotary cylinder (3). An enclosing outer surface (4) of the rotary cylinder serves as a wind-exposed surface in operation. So that the rotor sail can be operated in a more environmentally friendly manner, it is provided with a photovoltaic system (7) having solar cells (8) to generate electric energy for the drive (5) having an electric motor (6). The solar cells (8) are located in the rotary cylinder (3) and have photoelectrically active layers facing toward the enclosing outer surface (4). The rotary cylinder (3) has a sleeve (9) which is transparent, at least in an area covering the solar cells (8).