A hybrid powertrain unit includes an internal-combustion engine, and a gearbox device with a primary shaft that can be connected to the shaft of the internal-combustion engine via a clutch device. The gearbox device includes a secondary shaft with an output pinion meshing with a first crown wheel of a differential, the casing of which is rigidly connected to the casing of the gearbox device. An electrical machine is designed to function as electric motor and as electric generator, having a shaft connected by a transmission to a second crown wheel of the differential. An engagement device driven via an electronic control actuator is set between the shaft of the electrical machine and the second crown wheel. The electrical machine can be set coaxially to the output shafts of the differential or parallel thereto. Alternatively, the shaft of the electrical machine may be connected to the shaft of the internal-combustion engine by means of a belt transmission and engagement device.