The present invention relates to a run flat device for fitting to a tubeless wheel assembly for a motor vehicle, and to such a wheel assembly incorporating the device.A device of the invention comprises: an annular cover support structure subdivided into at least two ring sectors that are circumferentially juxtaposed; and locking means for locking the beads against the flanges to connect the support structure to the beads. According to the invention, the sectors are interconnected by connection means comprising a male member secured to one of the circumferential ends of each sector and removably mounted in a female member formed in the facing end of the adjacent sector, the male member comprising a projection that is terminated by an anchor tab, the female member being formed by a slot having a radially-lower zone and a radially-upper zone of widths that are respectively greater than and less the width of the tab, the upper zone being adapted to receive said projection axially locked in a position for anchoring the tab.