Product design for safe performance and operation in harsh environments triggers the importance of understanding property degradation due to prolonged exposure of advanced composite materials to such environments. The effect of seawater immersion on monotonic tensile, stress relaxation, and cyclic bending fatigue response of polyamide 66 nanocomposites containing 0%, 1%, and 5% hectorite organoclay were investigated. Seawater equilibrated nanocomposites demonstrated less moisture absorption compared to their pure counterparts. The imbibed seawater significantly increased the ductility, and reduced the tensile modulus and strength of nanocomposites. A nine-parameter Prony series viscoelastic constitutive model was used to estimate the influence of seawater on the viscoelastic behavior based on stress relaxation data. The phenomenological model evaluated a drop of 82% in the instantaneous and the residual elastic moduli indicating a significant plasticizing effect of seawater. However, the amount of increase in the rubbery nature of polyamide is observed from the reduction in stress relaxation time of first Maxwell element by about 40%. Nanocomposites exhibited improved fatigue life with an increase in organoclay content even after equilibration in seawater. The fracture surface observation of seawater equilibrated bending fatigue specimen revealed rubbery behavior indicating the positive difference between cyclic steady-state temperature and material glass transition temperature. © IMechE 2019.