A bicycle derailleur comprises a base member, a movable member, a pulley assembly, a rotational-force adjustment structure, an electric actuator, and a controller. The rotational-force adjustment structure is at least partly disposed between the movable member and the pulley assembly so as to apply resistance to rotation of the pulley assembly relative to the movable member. The electric actuator is configured to operate the rotational-force adjustment structure. The controller is configured to control the electric actuator based on a driving-force related information.