Both experimental tests and numerical simulations have been performed in this study to analyze the influence of long-term immersion (over 500 days) on the hygro-elastic and hygro-viscoelastic behaviour of epoxy resin samples. First, a diffusive and swelling campaign was carried out with water immersion at 25 °C and 60 °C. Various experimental tests have also been undertaken at different stages of ageing to highlight and quantify the influence of water diffusion on elastic and viscoelastic properties. A model is then proposed to simulate the hygro-viscoelastic behaviour of the studied material through finite element simulations. A good agreement between the experimental and numerical results is observed.