The sensing performance of a sol–gel-derived Pt/WO3 film used in an optical hydrogen gas sensor was evaluated and optimized. The Pt/WO3 film was deposited on a quartz glass substrate by the sol–gel method based on the acidification of Na2WO3 aqueous solutions. Optical transmittance changes of the films as a result of exposure to hydrogen-containing gas were measured in the near-infrared region. The sensor performance was severely affected by fabrication parameters such as annealing conditions, precursor concentration, and catalyst loading. In the case of annealing at 500 ℃, high sensitivity and fast response were obtained. An annealing time of at least 0.5 h was required to attain stable performance, and long annealing times had no effect on the sensitivity. However, the sensor performance and transparency of the film became poorer in the case of a ratio of over 0.4. The atomic ratio of Pt to W showed a considerable effect on the sensing characteristics. Both the sensitivity and response kinetics were promoted with increasing Pt loading. When the ratio was about 0.35, maximum sensitivity was observed. The concentration of the Na2WO4 solution used in the sol–gel process was also an important parameter. The sensitivity increased with increasing concentration since the thickness of the sensing film increased. The dependence of response speed on concentration was larger than that of recovery speed. When the film was exposed to H2/N2 gas, good response was observed; the hydrogen detection limit was about 0.001 vol% and minimal influence of water vapor was noted. The sensing performance, however, became poorer because oxygen inhibited the reaction when the film was exposed to hydrogen-containing air.