A series of poly(vinyl chloride) (PVC)/green coconut fiber (GCF) composites, with dioctyl phthalate (DOP) or thermoplastic polyurethane (TPU) as a plasticizer, were prepared by melt mixing. Their properties were studied in the molten state with an advanced nonlinear harmonic testing technique; in the solid state, the hardness and impact resistance were evaluated, and scanning electron microscopy was used for fractured surfaces. The effect of the fiber loading was investigated, as well as the role of the plasticizer. PVC-GCF composites are heterogeneous materials that, in the molten state, exhibit essentially a nonlinear viscoelastic character, in contrast to pure PVC, which has a linear viscoelastic region up to 50-60% strain. The complex modulus increases with the GCF content but in such a manner that the observed reinforcement is at best of hydrodynamic origin, without any specific chemical (i.e., permanent) interaction occurring between the polymer matrix and the fibers. As expected, PVC offers good wetting of GCFs, as reflected by the easy mixing and the rheological and mechanical properties. Fibers can be incorporated into PVC up to a 30% concentration without any problem, with the PVC/plasticizer ratio kept constant. Higher GCF levels could therefore be considered. Replacing DOP in part with TPU gives some benefit in terms of impact resistance, likely because of the viscoelastic nature of the latter and the associated energy absorption effects.