Poly(lactic acid) (PLA) has great potential in the plastic packaging industry due to its biodegradability, versatility, andgood performance. PLA compounding with different biobased materials to enhance the flexibility and biodegradability ofPLA-based packaging materials has attracted growing attention. Fungal biomass (FB), as a biological and eco-friendlyby-product stream from fermentation processes, can be a valuable blending component in PLA-based biocomposites.Therefore, in this study, 10 and 20 wt% of fungal biomass (FB), defatted fungal biomass (DFB), and fungal cell wall(FCW), as promising biobased materials, were added to PLA plasticised with 10 wt% triethyl citrate (TEC) to fabricatebiocomposites using the melt compounding technique. Our research focused on how the addition of fungal biomass andits fractions affected the mechanical and thermal properties, as well as the structure of PLA-based blends. These additionsdid not increase the strength of the biocomposites, but they did improve flexibility compared to regular plasticised PLA.The crystallisation of plasticised PLA samples blended with FB and DFB began at lower temperatures than neat PLA,plasticised PLA, and plasticised PLA blended with FCW. The addition of fungal biomass and its fractions, particularlyat 20 wt%, accelerated the biodegradation of PLA-based composites. Neat PLA and plasticised PLA degraded slowly insoil and retained most of their mass, whereas PLA-based blends containing FB and its fractions degraded significantlyfaster. It can be concluded that fungal biomass is a promising candidate for improving the flexibility of PLA blends andfacilitating their biodegradation in a natural soil environment.