Palladium (Pd) nanoparticles were prepared on photocatalytic TiO2-coated glass substrate by a photodeposition process and the optical hydrogen sensing properties were examined at room temperature. The TiO2 coatings were prepared on a non-alkaline glass substrate by a sol-gel process and hot water treatment. Pd nanoparticles were deposited on the TiO2 film by photodeposition using UV light. The obtained Pd/TiO2 thin film showed remarkable optical transmittance response to hydrogen gas at room temperature. The normalized transmittance of the Pd/TiO2 thin film at a wavelength of 640 nm decreased to 0.9 on exposing the film to hydrogen gas for only 5 s. This transmittance decrease is considered to be due to a gaschromic effect of the TiO2 photocatalytic coating.