The impact of partial water saturation on the mechanical behaviour of clays and claystones has been widely proved. This work aims at demonstrating that also for gas shales partial water saturation has a significant influ-ence on their stiffness and strength properties although they have different characteristics compared to other clayey materials. Samples extracted from two different unconventional shale gas reservoirs are tested. An experimental methodology and a testing device are developed to perform uniaxial compressive tests and unloading-reloading cycles at different total suction values. The control of total suction with the vapour equilibrium technique is used to equalize specimens to different water saturation conditions and assess the consequent impact on mechanical properties. The obtained outcomes clearly demonstrate that partial water saturation cannot be neglected to characterize the mechanical properties of gas shales. A reduction of uniaxial compressive strength of 22 % is observed in the total suction range between 150 and 10 MPa. Regarding the elastic stiffness, a decrease up to 50% is exhibited when the material is wetted from 150 MPa to 0 MPa of total suction.