The current study details on the moisture absorption behavior and its effect on the hybrid bionanocomposites of polylactic acid (PLA). In order to improve the compatibility between PLA and fiber, silanization was performed on fiber as well as C30B nanoclay was used as the secondary reinforcing filler. Silanization was confirmed through Fourier transform infrared study. In addition, thermogravimetric analysis (TGA) of fiber proved that hydrophilicity of fiber could decrease after silane treatment. Bionanocomposites of PLA were prepared using melt blending technique followed by injection molding. Samples were immersed in distilled water for 30 days at room temperature to analyze the moisture absorption behavior and its kinetic parameter. The results from moisture study revealed that PLA/fiber/nanoclay bionanocomposites have higher moisture resistance than PLA/fiber biocomposites. Further, the changes in mechanical as well as thermal properties of PLA and its composites during moisture absorption have been carried out. With increase in percentage of moisture absorption, the mechanical strength and modulus of composites decreased significantly, however, the unnotched impact strength and elongation at break found to improve. TGA of PLA and its composites revealed that thermal stability of composites decreased after moisture absorption. The morphology of the composites was monitored during moisture absorption, and the result revealed that moisture absorption has severely damaged the matrix–fiber interface.