The present invention relates to a run-flat device for attaching a tubeless mounted assembly for a motor vehicle, and to such a mounted assembly incorporating said device. A device (30) according to the invention includes: a tire support structure (30), to be mounted around a rim having a plurality of pieces and comprising a reinforcing frame, and a bead lock means for locking the beads against the rim edge and connecting the support structure to the beads. According to the invention, the frame comprises an axial belt having at least one pair of ends opposite each other, separated by a connecting axial interstice, and connected therebetween by detachable connecting means (35, 36) for locking the belt, the support structure being radially split on both sides on the right of each interstice to enable the manual mounting of the device inside the casing by the gradual insertion therein of the structure, temporarily free of the connecting means.