Abstract: A transmission assembly with braking device, particularly for vehicles, of the type comprising a speed reduction gear system between a driving shaft and a driven shaft. The braking device is constructed so as to be able to brake, on the one hand, the driving shaft and, on the other hand, the driven shaft. In a preferred embodiment, said driving shaft is coupled to the sun gear and said driven shaft to the planet gear carrier, the reaction ring gear for the planet gears being fixed. Said braking device then comprises a set of braking discs non-rotatably mounted but able to slide on a sleeve itself fixed on the driving shaft, and of which a part, through the action of an appropriate control, can come into frictional contact with the fixed discs integral with the casing, whereas the other part, through the action of said control, can come into frictional contact with the discs integral with said planet gear carrier.