Rock joints are formed at various stages and times, and have suffered from different alterations which affect the joint surface geometrical and mechanical properties. As a result of these alterations, joints will present different surface profiles on each side and different degrees of matching. It is common for joints to be equally rough but mismatched due to alternation and dislocation. A new joint matching coefficient (JMC) based on the percentage of joint surfaces in contact is proposed as an independent joint surface geometrical parameter. The JMC is coupled with the existing joint roughness coefficient (JRC) in order to fully describe the geometrical properties and to assess the hydromechanical behaviour of joints. It has also been demonstrated that joint matching is an important factor governing the aperture, normal closure, stiffness, shear strength and hydraulic conductivity of the joints. Therefore, JMC and JRC may be used together to estimate and predict the hydromechanical properties of rock joints.