Abstract Recently, there has been attractive attention over the defect-engineering where the optimum extent of oxygen deficiency has been considered as an effective route towards enhancing the solar-driven water oxidation photocurrent. In this study, a nanoplate-like tungsten oxide with dual oxygen and tungsten vacancies (WO3-x) was successfully fabricated on tungsten foil by acid-mediated hydrothermal treatment to highly improve the photoelectrochemical (PEC) performance of WO3-x photoanode. Thermal annealing at 550 °C in air led to an oxygen deficient surface with a sub-stoichiometric by phase transformation from orthorhombic WO3·.nH2O to γ-monoclinic. However, the optimum number of oxygen vacancies in WO3-x fabricated at 2 h (W-2 h) with compact, porous and uniform nanoplate film that provide a large surface area for efficient charge collecting capability, caused an enhanced photocurrent density of 4.12 mA/cm2 (41.2 mA/W) at 1.6 V vs. Ag/AgCl, as compared to W-3 h (2.59 mA/cm2, or 25.9 mA/W) and W-30 min (1.79 mA/cm2 or 17.9 mA/W). Finally, local variations in dual oxygen and tungsten vacancies and in the electronic band structure of a WO3-x thin film was analyzed with ultraviolet photoelectron spectroscopy (UPS) and UV–visible diffuse reflectance spectroscopy (UV-DRS).