In this work, we report on the synthesis of Ni-doped WO3 flakes by the hydrothermal method. The physical and chemical properties of the synthesized Ni-doped WO3 flakes were thoroughly investigated by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, Raman spectroscopy, photoluminescence spectrum, X-ray diffraction, X-ray photoelectron spectroscopy, and N2 adsorption–desorption measurement, thereby confirming the effect of Ni doping on H2S-sensing properties. The sensitivity to H2S gas of Ni-doped WO3 flakes showed short response/recovery times of 17 s/110 s, high stability, good selectivity, and low operating temperature of 250 °C. Due to the increasing surface defects and oxygen vacancies with the presence of Ni2+ ions in the structure of WO3 flakes, the Ni-doped WO3 sensor exhibited a better sensing of H2S gas than the pristine WO3 sensor. This result indicates that the Ni-doped WO3 flake structure is promising for detecting H2S gas as a selective, inexpensive, and outstanding sensor materials.