The present invention concerns a runflat device for a tubeless mounted assembly for a motor vehicle and an assembly incorporating said device. A device in accordance with the invention for a unit comprises a wheel rim having a plurality of pieces and a tire cover comprising beads mounted against the flanges of the rim, the device comprising: a support ring for mounting around the rim to support the tire cover following a drop in inflation pressure and divided into at least two ring sectors; and means for locking said beads against the flanges to connect the sectors to the beads. The device is such that each ring sector comprises a rigid wall which defines at least one internal volume and which defines the lateral faces of the ring sector.