The present invention relates to a puncture-resistant device for a non-pneumatic mounted assembly of a two-wheeled vehicle, in particular for a bicycle, and to such a mounted assembly incorporating this device. The invention applies to a mounted assembly with no inflation pressure, the tire of which is permanently supported by this puncture-resistant device.This puncture-resistant device (30) comprises an annular support (30), which is intended to be mounted between a rim (10) and a tire (20) of this mounted assembly (1) so as to permanently support this tire and which is constituted of a crosslinked rubber composition of cellular type having closed cells, the composition being based on at least one elastomer and comprising a reinforcing inorganic filler and at least one organic blowing agent.According to the invention, said composition comprises at least one active grade zinc oxide capable of activating the thermal decomposition of said blowing agent so as to stabilize the expansion of the support by giving it a minimized average density.