Novatein thermoplastic protein is bio-based alternative to petrochemical thermoplastics but depending on the application, may not have the appropriate mechanical properties. Titanium dioxide is an effective reinforcement for Novatein, but its polymorphs (rutile and anatase) behave differently as reinforcements. The effect of composition and polymorph type was investigated for Novatein composites containing up to 3 wt% TiO2. Additionally, the effect of water as a plasticiser was investigated by conditioning samples at 50% relative humidity, thereby removing some water that would otherwise act as a plasticiser. The unconditioned results (i.e. high water content) showed that the tensile strength of unreinforced Novatein increased by 20% and 16% for rutile and anatase, respectively whereas the secant modulus increased by 60% and 30%. After removing excess water, the tensile strength increased by 30% and 13%, whereas the modulus increased by 50% and 80% for each type. Impact strength showed the opposite behaviour and anatase generally lead to a greater increase. Impact strength increased by 15% and 70% for the rutile polymorh for high and low water content respectively, whereas it decreased by 30% and 40% for the anatase form. From these results, it can be understood that the rutile polymorh of TiO2 performed better with greater improvement in mechanical properties, most likely interacting differently with protein chains, leading to subtle changes in protein secondary structures.