The growing demand for sustainable materials has driven interest in polyhydroxyalkanoates (PHAs), such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), which are biodegradable alternatives to conventional plastics. PHBV's inherent brittleness limits its applications. To address this, natural waste-derived fillers, such as walnut shell (WS), cellulose (CEL) and wood filler (WF) were incorporated into PHBV matrices to enhance its mechanical and functional properties. The study investigated the impact of these fillers on PHBV composites, considering the mechanical performance of the developed composites and their hydrolytic degradation behaviour in a biologically active environment. The results indicated that the type and concentration of filler significantly influenced the material's performance. In general, the incorporation of fillers led to an increase in stiffness, accompanied by a reduction in tensile strength. The most pronounced changes were observed for composites containing CEL and WF, while less significant effects were noted for WS. The incorporation of 7.5 wt.% fillers altered the degradation behaviour of the mechanical properties, which was particularly evident in the case of the modulus. Degradation resulted in an overall decrease in mechanical performance, especially in tensile strength (by up to ~20%) and elastic modulus, with more pronounced effects in filler-reinforced composites compared to unmodified PHBV. In contrast, flexural strength exhibited relatively smaller changes, suggesting a partial preservation of structural integrity despite the progressing material degradation. This study underscores the potential of natural fillers for enhancing the sustainability and performance of PHBV composites, demonstrating their suitability for short-lifespan, compostable applications. These findings contribute to the development of fully biodegradable materials in line with circular economy principles, highlighting the importance of optimizing filler-matrix interactions for real-world applications. This study underscores the potential of natural fillers for enhancing the sustainability and performance of PHBV composites, demonstrating their suitability for short-lifespan, compostable applications. These findings contribute to the development of fully biodegradable materials in line with circular economy principles, highlighting the importance of optimizing filler-matrix interactions for real-world applications.