In this study, we report the successful fabrication of humidity sensors from zinc oxide (ZnO) nanorods (NRs) modified with platinum (Pt) nanoparticles (NPs) on a glass substrate through a hydrothermal method at 90 °C for 6 h. The crystal and optical properties of the pure ZnO and Pt–ZnO NRs were investigated through field-emission scanning electron microscopy (FE-SEM), X-ray diffraction analysis (XRD), photoluminescence (PL), and high-resolution transmission electron microscopy (HR-TEM). The structures of the humidity sensors demonstrated high photo-to-dark current contrast ratios, and fast response and recovery times for relative humidity (RH) measurement. The Pt NPs-adsorbed ZnO NR RH sensor showed better performance than that of the pure ZnO NR RH sensor. Results indicate that RH-sensing property can effectively be improved by adsorbed Pt NPs.