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The high cost of bioplastics compared to that of petrochemical-based plastics is an urgent challenge for widespread application, despite advances in bioplastic production technology. To secure price competitiveness, developing the complexation technology with additives that can partially replace PLA is inevitable. Mechanical properties of biodegradable plastic/biomass composites deteriorate owing to the surface properties and particle agglomeration in the composite. This deterioration can be mitigated by modifying the biomass surface through torrefaction; however, the mechanical strength of the composite may be reduced owing to the loss of cellulose. Wet torrefaction effectively improves the crystallinity of biomass by inducing the selective decomposition of hemicellulose while preventing the degradation of cellulose. In addition, it increases the hydrophobicity of the biomass surface while improving grindability by decomposing amorphous components and deconstructing a cell wall structure. Polylactic acid-based composites comprising modified wet-torrefied biomass exhibit improved tensile strength relative to composites containing untreated biomass. Therefore, biomass wet torrefaction is a promising strategy to prevent the deterioration of the mechanical properties of the composite while mitigating the limitations of dry torrefaction.