<p>This study explores the application of plastic optical fiber (POF) technology for rainfall-induced slope failure monitoring, focusing on the development and validation of a novel sensing system capable of capturing both hydraulic and structural changes in soil. The proposed system enables real-time detection of the early stages of slope instability by utilizing the optical sensitivity to changes in the volumetric water content and dry density of POF. A compact and low-cost experimental setup for light-intensity monitoring was developed by integrating a refraction–reflection-based POF sensor with a simple light source and an optical signal analysis unit. To evaluate the system’s performance, a series of control calibrations and model slope tests were conducted using sandy and silty soils. As a result, it was confirmed that the reflected light intensity captured by the sensing interface at the probe tip is jointly influenced by both water infiltration and density disturbances, and that deviations from the expected signal trend can serve as an early indicator of slope failure. The system exhibited distinct response patterns under various failure mechanisms, successfully capturing both rapid sliding and gradual erosion processes. Beyond the experimental results, this study highlights the advantages of POF sensing, including high sensitivity, environmental robustness, cost-effectiveness, and the potential for integration with machine learning and multiphysics frameworks. These features make this technology particularly suitable for long-term landslide monitoring and early warning in shallow slope environments. Overall, POF-based sensing systems offer a promising alternative to existing geotechnical monitoring tools and have high potential for practical application in hazardous areas.</p>