The present study aims to investigate the water absorption property and its effect on the mechanical properties (i.e. tensile, flexural, and impact) of bionanocomposites. The epoxy-based bionanocomposites were prepared by reinforcing the epoxy with 1, 2, 3, 4, and 5 wt% of nanocellulose using in situ polymerization method. The maximum water uptake by bionanocomposites was measured; however, the water absorption behavior was not found in accordance with Fickian’s diffusion model. In the present study, very low water absorption in the range of 0.17–0.34% was offered by bionanocomposites. The results obtained from the present experimental study suggested that there were a maximum degradation of 14.96% in tensile strength, 26.44% in flexural strength, and 55.66% in impact strength for bionanocomposites reinforced with 5 wt% of nanocellulose by water uptake.