Rising oil, gas and plastic prices, limited landfill space, and health and environmental concerns all point to a need for plastics derived from renewable resources. Soy protein plastics could meet this need. Soy protein plastics literature shows these materials can be manufactured on conventional plastics processing equipment. However, viable processing requires novel formulations. Additionally, the mechanical, dimensional stability and water resistance properties of soy protein plastics must be improved to compete with the properties of current synthetic commodity resins. Characterization of novel soy protein-cornstarch blends and natural fiber composites of these blends has not been reported previously. This dissertation explores formulation, processing, and properties of soy protein plastics. A variety of soy protein plastic formulations were created and screened using statistical methods to determine significant factors affecting mechanical properties, viscosity, dimensional stability and water resistance. Results of experiments utilizing compression molding, batch and continuous compounding processes, injection molding, extrusion, capillary rheometry and thermoforming are reported. Soy protein-cornstarch blends were found to exhibit shear thinning and viscosity behavior similar to low-density polyethylene and polystyrene. Several non-formaldehyde crosslinking agents were found to not significantly improve properties of compression molded samples. Cornstarch and sodium sulfite were found to improve processability and tensile properties (stress, modulus and elongation at break) of injection molded soy protein plastics. Cellulose fiber reinforcement produced significant gains in peak stress, modulus and dimensional stability while preserving an acceptable level of elongation at break. A titanate coupling agent yielded insignificant improvements in properties. An annular extrusion die for the production of tubular profiles was designed using measured viscosity data. Tubular profiles of soy protein plastic with good surface quality were successfully extruded for the first time. Water resistance of all tested materials was poor. However, even the current materials may be appropriate for applications that require fast decomposition in water. For better water resistance, other formulations (e.g., blending with PLA) or coatings (including zein) need to be explored. Recommendations for future research, including thermoforming, are made.