A rotor blade (20), especially for a rotary wing aircraft, has an aerodynamically effective rotor blade profile which comprises a profile tip section (21), a profile base (20a) with a profile core (22) and an upper and lower covering skin (30) enclosing the profile core (22), and a profile trailing edge section (23) with a trailing edge (40). A reversibly flexible flection actuator (26) is fastened with a first end to an end section of the profile base (20a) pointing towards the trailing edge (40) and projects with a second end from the profile base (20a) and the end section thereof towards the trailing edge (40). The flection actuator, with the projecting second end, forms a part of the profile trailing edge section (23) of the rotor blade profile and a mobile rotor blade flap (24) which can be deformed to an arched rotor blade flap deflection when the flection actuator (26) is bent.