The mechanical behaviour of the hygrothermal aged carbon fibre reinforced epoxy composite material was studied by performing quasi-static tensile, compressive and three-point bend tests. Hygrothermal ageing of the composite material was done by immersing it in natural seawater at 60 °C. The resulting moisture diffusion into the material was explained using the Fick and Langmuir diffusion model. The variation of the strength, modulus and failure strain of the material with respect to the absorbed moisture content was established. An empirical model adopted from the literature was modified to describe the experimental variation of the strength of the material as a function of the absorbed moisture content. The fitness of this current empirical model was compared with other models from the literature. Finally, the statistical distribution of the dry and hygrothermal aged material strength data was established using the two and three-parameter Weibull distributions, respectively. The distribution parameters of the hygrothermal aged material strength were derived as a function of the absorbed moisture content, residual strength and the distribution parameters of the dry material strength.