Torrefaction is an effective thermochemical process for modifying the properties of lignocellulosic fibers, enhancing their compatibility with polymer matrices. Herein, the effects of wet torrefaction on the surface chemistry of hemp fibers (HFs) and their compatibility with the polyhydroxyalkanoate (PHA) polymer matrix were investigated. Results showed that torrefaction considerably changed the physicochemical properties of HF, leading to improvements in hydrophobicity, mechanical reinforcement, and thermal stability. Increasing the torrefaction temperature resulted in carbon enrichment and crystallinity enhancement while reducing hemicellulose content and volatile components. These modifications contributed to increased fiber rigidity, enhanced interfacial compatibility with the PHA matrix, and improved mechanical properties of the composite. The optimal torrefaction temperature was identified at 200 °C, where fiber individualization and surface roughness were maximized, leading to superior interfacial bonding with PHA. The resulting torrefied HF–reinforced composite, PHA/HF-200, exhibited a significant increase in tensile modulus (211 %), tensile strength (55 %), flexural modulus (361 %), and flexural strength (329 %) compared to neat PHA. In addition, torrefaction enhanced the moisture resistance of composites, reducing water absorption and improving dimensional stability under humid conditions. Despite delayed initial biodegradation due to increased hydrophobicity, complete biodegradation was achieved within 40 days, confirming the environmental sustainability of the PHA/HF-200 composite.