This paper shows how physical ageing and plasticization processes are coupled in an amine-based epoxy. The study combines several ageing conditions including thermal ageing in the dry state (from 40 to 60 °C), ageing in a humid environment (RH 50% and RH 75%) as well as immersion in seawater at several temperatures from 15 to 40 °C. First results are presented to highlight that the two processes (physical ageing and plasticization) both occur when the epoxy is immersed in water. A step-by-step approach is then used to evaluate how the plasticization process induced by the presence of water is affected by physical ageing and vice versa. It appears that the plasticization process, which leads to a large decrease in maximum stress under tensile loading, is not affected by physical ageing of the epoxy. The physical ageing mechanism is not affected by the presence of water in the polymer, i.e. the same increase in maximum tensile stress (26 MPa) is observed in air and in water. However, the kinetic rate of physical ageing is much faster in water due to the plasticization of the polymer. These effects must be considered when accelerated aging methods are applied.