Present days, the world needs more sustainable and eco-friendly materials to replace and reduce the use of synthetic materials to prevent pollution. In this regard, the agro-waste after soy cultivation is identified as a sustainable resource of raw material and can be used as reinforcement in polymer matrices. The current research focuses on the effect of surface modification in agro-waste soy stem fibers and their composites. The fibers were collected, cleaned, processed, and subjected to chemical treatment. The chemical analysis showed an increased ratio of cellulose % in the soy fibers when compared to untreated fibers. The silane treatment showed improvement in the thermal stability of soy fibers and reduced the coefficient of thermal expansion in silane treated composite. The X-ray diffraction test shows that there was a 22.57% increase in the crystalline index due to oxalic acid treatment. The chemical treatments also influenced the crystal size of the soy fibers which had direct impact on the water absorption nature of the developed composites. The mechanical performance of the composites is evaluated through tensile, flexural, impact, and interlaminar shear strength according to the ASTM. The silane treated fibers in the composites showed a better mechanical performance compared to untreated and other chemical treatment. However, oxalic acid treatment showed better mechanical performance when compared to traditional NaOH treatment. The increase in wettability nature influenced the mechanical strength and water-resistant nature of the composite. The complete results state that the silane treatment on the soy stem fiber showed the best performance followed by the oxalic acid and alkali treatment when compared to the raw fibers. From the results, it is observed that the Agro-waste soy fiber is a sustainable and renewable resource of raw material that can be used as reinforcements in polymer matrices for lightweight structural applications.