In this work, high density polyethylene (HDPE)/hemp fiber composites were produced by extrusion compounding followed by injection molding. In particular, the effect of ground tire rubber (GTR) and coupling agent (maleated polyethylene, MAPE) content was studied to modify the mechanical properties of these natural fiber composites (NFC). From the samples produced, a complete characterization was performed including morphology, density, hardness, as well as mechanical properties in tension, flexion, and impact. The results indicate that substantial improvement in NFC impact strength occurred after GTR addition, while tensile and flexural moduli/strengths decreased. On the other hand, the addition of a coupling agent was able to improve adhesion between each phase resulting in better composite properties. Overall, the final properties of the composites represent a balance between elasticity/toughness from GTR and rigidity/strength from hemp and MAPE. From the data obtained, a regression model for different properties is presented to design/control the final properties of these composites.