A reinforcement for a boot, in particular a sports boot, more particularly a cross-country ski boot. The boot reinforcement makes it possible to improve the torsional stiffness, efficiency, durability, lightness, cost, foot protection, and industrial workability. To achieve this goal, the boot reinforcement according to the invention includes a front zone A from the front end up to the beginning of the plantar arch, a median zone B corresponding to the plantar arch, a rear zone C. The reinforcement includes, at least in the zones B and C, at least one "sandwich" structure constituted by at least one core inserted between at least two layers, and, in the zone A, it is flexible in a substantially longitudinal direction and torsionally stiff. The layers of the sandwich structure are made of composite, carbon fibers/polymeric resin, whereas the core of this sandwich structure is made of synthetic foam, wood or honeycomb. Each zone A, B, C has a longitudinal flexural strength RfA, RfB, RfC, such that RfA