Abstract: A plantar orthosis is presented for supporting a user's plantar arch, in which a heel portion and a metatarsus portion are identified, separated by an intermediate portion of the plantar arch. The orthosis includes: a covering layer configured to receive a user's resting foot; a finishing lower layer; a base reinforcement layer, intermediate between the covering layer and lower layer, supporting and guiding dynamic motions of compression, extending and twisting the plantar arch in the non-linear action thereof of helical winding/unwinding of pronosupination; and a dynamic element, positioned between the base reinforcement layer and the lower layer, which is formed by a continuous belt-like strip, arranged near an external edge of the heel portion, and which extends with one or more belt-like branches as far as respective front resting portions of the plantar orthosis at an external midfoot and proximate a hallux of the plantar arch.