An experimental investigation to analyze the effect of suction on the Poisson's ratio of claystone is presented. For this, the vapor transfer technique is used to impose suction on rock specimens. To evaluate the mechanical behavior of the claystone, two conditioned specimens are tested at each suction value through uniaxial compressive tests. Axial and radial strains are analyzed using the moving point regression technique. Axial and radial stiffness increase with suction and exhibit a non-linear behavior during the loading process. Poisson's ratio was found to decrease with suction and to evolve in a non-linear manner during loading.