Photoelectrochemical water splitting is a promising approach for the carbon‐free production of hydrogen from sun light. Here, robust and efficient WO3 photoanodes for water oxidation were synthesized by scalable one‐step flame synthesis of nanoparticle aerosols and direct gas‐phase deposition. Nanostructured WO3 films with tuneable thickness, bandgap and controllable porosities were fabricated controlling the aerosol deposition time, concentration and temperature. Optimal WO3 films demonstrates superior water oxidation performance reaching a current density of 0.91 mA at 1V vs. Ag/AgCl and an IPCE of ~ 61% at 360 nm in 0.1 M H2SO4 solution. Notably, it is found that the excellent performance of these WO3 nanostructures arises from the high in‐situ restructuring temperature (~1000 °C), which increases oxygen vacancies and decreases charge recombination at the WO3/electrolyte interface. These findings provide a scalable approach for the fabrication of efficient WO3 and other metal oxides‐based photoelectrodes for light‐driven water splitting.