A hybrid powertrain unit comprises an engine and a gearbox device with a primary shaft that can be connected to a shaft of the engine via a clutch. The gearbox device comprises a secondary shaft with an output pinion meshing with a first crown wheel of a differential, the casing of which is rigidly connected to a casing of the gearbox device. An electric machine of the unit is configured to function as electric motor and an electric generator, having a shaft connected by a transmission to a second crown wheel of the differential. In the transmission, which is arranged between the electric machine shaft and the second crown wheel is an engagement device that can be driven via an actuator. The electric machine shaft is connected to the engine shaft on a side opposite of the gearbox device. The transmission includes a gear-reduction jump constituted by a belt transmission.