Seasonal variations in the soil moisture content can result in significant changes in soil suction. Rainfall infiltration can cause loss of matric suction in the unsaturated lateritic soil. Several failures are reported in lateritic soil slopes in Kerala, associated with rainfall. Geosynthetics are generally used for improving the stability of soil structures constructed with poorly draining soils. Shear strength at the interface of unsaturated lateritic soil with geosynthetics plays a crucial role on the internal stability of geosynthetics reinforced structures. The present study investigates the influence of rainfall-induced wetting on shear strength of lateritic soil and lateritic soil-geosynthetics interface. Lateritic soil was collected from a site in Kerala, which was subjected to rainfall-induced slope failure. Geotechnical characterization of the soil was carried out. Shear testing was conducted on soil samples of size 305 × 305 × 200 mm. When the moisture content was increased by 4% due to wetting, the shear strength of the lateritic soil was reduced by 20%. In contrast, the corresponding reduction in strength at the interface for the soil–geosynthetic system was only 3%. Similarly, an increase in moisture content by 8% due to wetting resulted in a reduction in strength by 30% and 4% for soil–soil and soil–geosynthetic systems, respectively.