Herein described is a gearmotor assembly for bicycles comprising a pedal crank shaft, an electric motor provided with an electronic control unit, a reduction gear mechanism and a gear wheel, arranged coaxially around the pedal crank shaft. The gearmotor assembly further comprises two internal overrunning clutches, mounted coaxially around the pedal crank shaft and configured to transmit torque in a first direction of rotation of the pedal crank shaft, and an external overrunning clutch, mounted coaxially inside the gear wheel and configured to transmit torque in a second direction of rotation of the pedal crank shaft which is opposite with respect to the first direction of rotation. A hollow tubular element kinematically connected to the drive wheel for transmitting the torque is interposed between the internal and external overrunning clutches. The internal overrunning clutches are housed by interference inside the hollow tubular element, while the external overrunning clutch is housed externally by interference. The pedal crank shaft is provided with two opposite rectified seats, around each of which a respective internal overrunning clutch is mounted by interference. The internal overrunning clutches are then arranged directly at contact with the pedal crank shaft. The hollow tubular element forms the outer housing of the internal overrunning clutches and it consists of a rectified tube around which the gear wheel, within which the external overrunning clutch is coaxially mounted, is centrally housed.