The reuse of soybean oil reduces environmental impacts, prevents sewage system clogging, promotes sustainability, and adds value to a waste material. In this study, post-consumer epoxidized soybean oil (RESO) was cured with fumaric acid to develop green composites reinforced with sugarcane bagasse (F). The natural reinforcement underwent chemical modification to improve its interaction with RESO, aiming to evaluate impact strength, tensile strength, Shore D hardness, and contact angle. Additionally, scanning electron microscopy (SEM) was used to analyze the interfacial interaction between the RESO matrix and the sugarcane bagasse before (F) and after treatment (TF). The RESO/F composites with 10 and 20 parts per hundred resin (phr) did not exhibit a reinforcement effect; they acted only as traditional fillers, improving only the elastic modulus and Shore D hardness compared to pure RESO. In contrast, the treatment of sugarcane bagasse contributed to the production of RESO/TF composites with improved mechanical performance. The RESO/TF (20 phr) formulation showed increases of 230.7% in elastic modulus, 64.2% in tensile strength, and 114.7% in Shore D hardness compared to pure RESO, suggesting a reinforcement filler effect. SEM analysis revealed that the RESO/TF (20 phr) composite exhibited improved interfacial interaction between the phases, which enhanced stress transfer as observed in the mechanical tests. The contact angle of approximately 95.4° and the impact resistance of 38.3 J/m were similar to those of the RESO matrix, reinforcing that the chemical treatment of the sugarcane bagasse contributed to a synergistic effect on the material properties. The reuse of RESO for composite production aligns with circular economy principles and sustainability.