Abstract: The weight transfer device is for a tractor trailer including a tractor for pulling a trailer. The device comprises a mount having opposite ends provided with attaching elements for attaching transversely the mount between a pair of rearward parallel frame side rails of the tractor behind its fifth wheel. The mount receives the base of a bag unit having an upper surface over the base at an adjustable distance therefrom, and an inflatable air bag between the base and the upper surface for adjusting the distance. A bearing structure is mounted on top of the upper surface for upwardly pushing against the trailer without significantly hindering its pivotal movement around the fifth wheel. The device further comprises a compact guiding mechanism for vertically guiding the upper surface of the bag unit while preventing the bag from drooping when the bearing structure is pushing against the trailer.