A gearmotor assembly for a bicycle comprising at least one frame, at least one drive wheel and at least one pedal crank/pedal assembly, the gearmotor assembly comprising: - at least one pedal crank shaft movable by rotary motion around a predefined axis, said at least one pedal crank shaft being integrally joined with the pedal crank/pedal assembly for receiving the torque delivered by the cyclist through said pedal crank/pedal assembly and for transmitting said torque to said at least one drive wheel; - at least one electric motor provided with an electronic control unit; - at least one reduction gear mechanism (18, 20) comprising a worm screw, integrally joined with the output shaft of said at least one electric motor, and a gear wheel, arranged coaxially around said at least one pedal crank shaft, said at least one reduction gear mechanism (18, 20) being configured to receive the torque delivered by said at least one electric motor and to transmit said torque to said at least one drive wheel by means of said at least one pedal crank shaft; - at least one internal overrunning clutch, mounted coaxially around said at least one pedal crank shaft, and configured to transmit torque in a first direction of rotation of said at least one pedal crank shaft, and - at least one external overrunning clutch, mounted coaxially in said gear wheel and configured to transmit torque in a second direction of rotation of said at least one pedal crank shaft which is opposite with respect to said first direction of rotation, wherein at least one hollow tubular element kinematically connected to said at least one drive wheel for transmitting said torque is interposed between said at least one external overrunning clutch and said at least one internal overrunning clutch, the gearmotor assembly being characterised in that said at least one internal overrunning clutch is housed by interference inside said at least one hollow tubular element and said at least one external overrunning clutch is housed by interference outside said at least one hollow tubular element, wherein said at least one pedal crank shaft is provided with two opposite machined seats, around each of which a respective internal overrunning clutch is mounted by interference, thus, said gearmotor assembly being provided with two distinct internal overrunning clutches arranged directly at contact with said at least one pedal crank shaft, and wherein said at least one hollow tubular element forms the outer housing of said two internal overrunning clutches and it consists of a machined tube that centrally holds, that is in a median position between said two internal overrunning clutches, said gear wheel inside which a single external overrunning clutch is coaxially mounted, so that said single external overrunning clutch is interposed between said gear wheel, which surrounds it, and said at least one hollow tubular element. - at least one pedal crank shaft movable by rotary motion around a predefined axis, said at least one pedal crank shaft being integrally joined with the pedal crank/pedal assembly for receiving the torque delivered by the cyclist through said pedal crank/pedal assembly and for transmitting said torque to said at least one drive wheel; - at least one electric motor provided with an electronic control unit; - at least one reduction gear mechanism (18, 20) comprising a worm screw, integrally joined with the output shaft of said at least one electric motor, and a gear wheel, arranged coaxially around said at least one pedal crank shaft, said at least one reduction gear mechanism (18, 20) being configured to receive the torque delivered by said at least one electric motor and to transmit said torque to said at least one drive wheel by means of said at least one pedal crank shaft; - at least one internal overrunning clutch, mounted coaxially around said at least one pedal crank shaft, and configured to transmit torque in a first direction of rotation of said at least one pedal crank shaft, and - at least one external overrunning clutch, mounted coaxially in said gear wheel and configured to transmit torque in a second direction of rotation of said at least one pedal crank shaft which is opposite with respect to said first direction of rotation,