A composition for forming a reinforced composite siding product and a composite siding product formed by an extrusion process utilizing the composition is provided. The mixed resin formulation utilized to form the composite siding product includes a polymeric resin, a filler, a processing aid, at least one lubricant, and a thermal stabilizer. In a preferred embodiment, the polymer resin is polyvinyl chloride and the filler is talc. The mixed resin formulation may be extruded with a reinforcement material in a screw extrusion process to form a composite siding product. One or more reinforcing fibers may be utilized in the extrusion process. The reinforcement fibers may be present in the final product in an amount up to 25% by weight of the final product. In at least one preferred embodiment, the reinforcement fibers are glass fibers. A weatherable cap may be co-extruded to form the final reinforced, foamed composite siding product.