Stemflow serving as point input to the forest floor could have great implications for soil water dynamics and slope stability around a tree. To clarify the effect of stemflow on soil water dynamics and slope stability, we conducted field observations and numerical simulations of rainwater infiltration processes on a forested hillslope. The results of field observations indicated that locally concentrated rainwater input attributable to stemflow on the downslope side of the tree trunk caused the large and rapid increases in water content and pore water pressure in the region downslope of the tree stem, resulting in the development of an asymmetric saturated zone around the tree. Rainwater infiltration simulations were conducted with the proposed model (Liang et al., 2009 a), in which stemflow was parameterized as a source flux spring in soil layers. The simulation results were then used for the slope stability analysis. The proposed model showed more rapid decreases in the minimum safety factor attributable to greater generation rates of saturated zones downslope of trees. This model also resulted in the timing of minimum safety factor < 1.0 at an earlier stage than in the conventional model. Furthermore, the proposed model showed evidence of the risk for slope failure throughout all parts of the slope, though the conventional model showed only the risk of slope failure in the lower part of the slope.