A hybrid powertrain unit comprises an internal-combustion engine (E) and a gearbox device (C) with a primary shaft (3) that can be connected to the shaft (5) of the internal-combustion engine (E) via a clutch device (6). The gearbox device (C) comprises a secondary shaft (7) with an output pinion (9) meshing with a first crown wheel (10) of a differential (D), the casing (11) of which is rigidly connected to the casing (2) of the gearbox device (C). The unit further comprises an electric machine (M) adapted to operate both as electric motor and as electric generator, having a shaft (16) connected by means of a transmission to a second crown wheel (19) of the differential (D). In the connecting transmission, arranged between the shaft (16) of the electric machine (M) and the second crown wheel (19) of the differential (D) is an engagement device (25) that can be operated via an electronically controlled actuator. The transmission that connects the shaft (16) of the electric machine (M) to said second crown wheel (19) of the differential (D) includes an auxiliary shift device configured for providing at least two different selectable transmission ratios.