Opalinus Clay is a fine-grained sedimentary geomaterial with a mineral composition consisting mainly of silicates, carbonates and quartz. Due to its favourable barrier properties, this geomaterial has been selected as the host rock for the underground disposal of radioactive waste in Switzerland. In this context, the hydro-mechanical state of Opalinus Clay will be affected by subsequent drying and wetting phenomena. Design procedures require then an appropriate understanding of the processes related to water exchange. To analyse the behaviour of Opalinus Clay upon suction changes and to define its water retention properties, the vapour equilibrium technique is combined with an accurate assessment of the deformations in the two orthogonal directions using strain gauges. Based on those results, a 3D Thermo-Hydro-Mechanical Finite Element model is implemented and validated. Preliminary analysis shows overall a good agreement between experimental data and modelling results in terms of change in water content, degree of saturation and equilibrium time. The numerical model will be used to define an accurate law for the tortuosity and permeability evolution of Opalinus Clay when subjected to suction variations, that is required to simulate the behaviour of a nuclear waste disposal system.