Epoxy resins are one of the most commonly used class of materials in polymeric composite industry. One of their main weak points is their vulnerability to moist and hot environments. In this paper, the effect of hydrophilic Fe2O3 nanoparticles, hydrophobic graphene-oxide nanoplatelets (GONPs) and a hybrid of both was experimentally studied on the moisture absorption and mechanical properties degradation of the nanocomposites. Different weight percentages of nanofillers were used for fabricating the nanocomposite specimens. The neat and nanocomposite specimens were aged in hot deionized water. The gravimetric test results revealed that the Fe2O3 nanoparticles and GONPs caused 12.4 and 6.7% reductions in the ultimate moisture uptake of the nanocomposites, respectively. The addition of 1 wt % Fe2O3 nanoparticles reduced the moisture diffusion coefficient by 11.7% due to the increased polarity of epoxy, while 0.25 wt % GONPs increased it by 38.5% due to the increased free volume of epoxy. Moreover, the wet Fe2O3/epoxy nanocomposites experienced the maximum 28.8 and 52.9% higher strength and stiffness compared to the wet neat epoxy, respectively. GONP/epoxy nanocomposites showed the maximum 22.3 and 13.4% higher values of strength and stiffness compared to the wet neat epoxy.