Abstract: A thermoplastic composite reinforcing sheet material useful for reinforcing orthotic devices and other articles, such as a prosthetic socket or a ski boot. The composite sheet material consists of a plurality of layers. Two layers form outer opposed sides of the sheet and are made from a thermoplastic material. The core of the sheet consists of a plurality of fiber-containing layers. Each layer is impregnated with a thermoplastic resin which will bond with the outer thermoplastic layers when the final shape is vacuum thermoformed. The orientation of the fibers in the core may be varied to change the strength and rigidity of the composite thermoplastic reinforcing sheet. An orthotic device can be built using a thermoplastic host material and the disclosed thermoplastic composite reinforcing sheet material. The host material defines the overall shape of the orthotic device and is any thermoformable thermoplastic. The reinforcing sheet is used to reinforce desired sections of the final orthotic device.