An electric derailleur motor unit includes a base member, a motor, an output shaft, and a drive train. The motor is mounted to the base member. The motor has a motor shaft rotatable about a first rotational axis. The output shaft is rotatable about a second rotational axis. The drive train is operatively disposed between the motor shaft of the motor and the output shaft. The drive train includes an anti-backdriving device. The anti-backdriving device is configured to transmit rotation of the motor shaft of the motor in both rotational directions about the first rotational axis to the output shaft. The anti-backdriving device is further configured to prevent the output shaft from rotating in both rotational directions about the second rotational axis while the output shaft receives an external rotational torque from outside of the electric derailleur motor unit.