A bicycle derailleur comprises a base member, a movable member, a motor unit, and a controller. The controller is configured to control the motor unit to rotate an output shaft of the motor unit at a first maximum voltage during a first shifting operation of the chain in a first shifting direction. The controller is configured to control the motor unit to rotate the output shaft of the motor unit at a second maximum voltage during a second shifting operation of the chain in a second shifting direction. The first maximum voltage is different from the second maximum voltage. The bicycle derailleur is configured to receive electricity from an electric power source configured to supply electricity to an assist driving unit.