In this study, we demonstrated a hydrogen (H2) sensor based on a guided-mode resonance (GMR) filter coated with a layer of palladium (Pd). The sensor consists of three key layers, namely the substrate (replicated grating structure on an optical adhesive), waveguiding layer of TiO2, and active-sensing layer of Pd. The sensor functions as an optical resonator. On absorption of H2, the change of the refractive index in Pd changes the optical response of the sensor. To monitor the output optical response, a transmission setup was used, and two detection modalities-the change in transmission efficiency at a specific wavelength and the shift in the resonant wavelength-were demonstrated. Upon injection of a mixture of 4% H2 and 96% argon, the resonant wavelength shifted 6.38 nm, and transmission efficiency increased by 45% in 24 s.