A bicycle electric/electronic derailleur has mutually movable components, and a geared motor for controlling the mutual motion of the components. The geared motor includes an electric motor having a drive shaft, an output shaft and a speed reducer including at least one gear between the drive shaft and the output shaft. The output shaft defines at least in part one of the articulation axes. The electric motor is rigidly connected to a first component, and a second component integrally rotates with the output shaft. The derailleur further includes a magnetic rotary encoder for transducing a quantity related to the mutual motion of the components. The encoder includes a permanent magnet and a sensor responding to the changes in the magnetic field of the magnet. The magnet is a disc, cylindrical or annular magnet having diametral magnetization, and is coaxial with and arranged to integrally rotate with the output shaft.