The effect of water saturation on the mechanical properties of various sedimentary rocks of different lithologies is investigated. Experimental results show that the uniaxial compressive strength and Young’s modulus of a rock decreases with increasing water saturation. Moreover, water-flooding of low saturation rocks under constant stress conditions reveals a significant increase in rock compressibility when the rock is close to its plastic yield. A theoretical model based on capillary suction and Bishop’s effective stress principle for partially-saturated materials is presented and used for the calculation of the uniaxial compressive strength of the rock as a function of water saturation thus verifying the importance of the proposed mechanism.