The accumulation of post-consumer polyethylene terephthalate (PET) waste necessitates value-added recycling strategies that restore mechanical performance while maintaining environmental sustainability. In this study, recycled PET (rPET) was reinforced with 20 wt% short jute fibres subjected to untreated (UT), NaOH-treated (NT), and silane-treated (ST) surface modifications. Composites were fabricated by injection moulding and characterised using X-ray diffraction and FTIR to confirm removal of amorphous constituents and formation of interfacial siloxane linkages. Surface treatment improved consolidation, with experimental density increasing from 1.22 to 1.24 g/cm³ and porosity decreasing from 3.9% to 2.4%. Correspondingly, tensile strength increased from 38.78 to 48.29 MPa, flexural strength from 50.30 to 68.84 MPa, short-beam shear strength from 11.74 to 14.66 MPa, and hardness from 77 to 81 Shore D. One-way ANOVA confirmed statistically significant improvements (p < 0.05). Porosity-based linear regression modelling demonstrated strong structure–property relationships (R² = 0.631–0.916), with cross-validated R² values of 0.391–0.861, indicating moderate predictive robustness within the investigated domain. A cradle-to-gate life cycle assessment revealed low carbon footprints (2.49–2.51 kg CO₂-eq kg⁻¹), with fibre surface modification introducing only marginal environmental penalties. The results demonstrate that surface-engineered jute fibres effectively enhance the mechanical integrity of recycled PET while maintaining favourable sustainability performance.