Landslides and slope failures in natural clay-rich materials are frequently observed in coastal areas and other areas where these materials were deposited in a saline water environment. High sodium chloride (NaCl) concentration in the pore fluid results from deposition in this environment. As shown by the results of this study, subsequent reduction in the NaCl concentration results in reduction in fully softened shear strength, with potentially significant reduction in slope stability. Fourteen sample materials were collected from seven different landslide areas in Niigata Prefecture, Japan, to evaluate the effect of saline water on fully softened shear strength (FSSS). The sample materials were collected from the intact mudstones and the sliding surface materials. Pore waters from the powdered sliding surface materials exhibited low concentrations of NaCl; however, the pore waters in the powdered intact rock samples exhibited a high concentration of NaCl. Washing of NaCl from the powdered intact rock samples caused a significant drop in FSSS, whereas mixing the sliding surface materials with NaCl at 0.5-M concentration increased the FSSS of the samples significantly. To evaluate the effect of saline water on various types of clay minerals, 12 samples—nine with montmorillonite as the clay mineral and three with kaolinite as the clay mineral—were prepared in the laboratory, and Atterberg limits and FSSS of these mineral mixtures were measured. There was a significant increase in the FSSS when the mineral mixtures with more than 9.5% montmorillonite were tested with saline water. The effect was negligible or negative for mineral mixtures having 9.5% or less montmorillonite content and any amount of kaolinite. An excellent correlation was observed between the liquid limits, plasticity indexes, and corresponding fully softened friction angles irrespective of the type of pore water. Likewise, there was a good correlation between the ratios of the liquid limits and plasticity indexes measured with different pore waters and the corresponding ratios of the fully softened shear strengths. It was observed that the reduction in FSSS of powdered intact rock with NaCl leaching could reduce the calculated factor of safety of the landslides by more than 0.3, demonstrating the negative effect of NaCl leaching from the slope materials on the stability of slopes.