Sesame husks were used as filler, for the first time, to reinforce unsaturated polyester resin matrix composites. The sesame husk particles were subjected to an alkali treatment with 5% NaOH solution and were characterized with XRD and SEM. Composites were fabricated with untreated as well as alkali-treated sesame husks (33 wt % filler loading) and were characterized for their flexural properties, impact strength, dynamic mechanical properties, fracture behavior, water absorption characteristics, and thermal degradation behavior. The flexural strength, flexural modulus, and impact strength increased by 30%, 40%, and 46%, respectively, in the alkali-treated sesame husk-reinforced composites compared to those of untreated sesame husk-reinforced composites. Here, the increase in flexural strength properties was accompanied with increased impact strength, which is a unique feature of these composites. Water absorption was found to be 3.9% after 7 days in both the untreated as well as alkali-treated sesame husk-reinforced composites.