Composite reinforcer (R-2) that is self-adhesive by curing to a diene rubber matrix, which can be used as reinforcing element for a pneumatic tire, comprising: one or more reinforcing thread(s) (20), for example a carbon steel cord; covering said thread, individually each thread or collectively several threads, a layer of a thermoplastic polymer composition comprising on the one hand a thermoplastic polymer, the glass transition temperature of which is positive, on the other hand a functionalized unsaturated thermoplastic stirene elastomer, the glass transition temperature of which is negative, said elastomer containing functional groups selected from epoxide, carboxyl and acid anhydride or ester groups. Process for manufacturing such a composite reinforcer and rubber article or semi-finished product, especially a pneumatic tire, incorporating such a composite reinforcer.