Here we report that photostability and photoactivity of WO3 for water oxidation can be simultaneously enhanced by controlled introduction of oxygen vacancies into WO3 in hydrogen atmosphere at elevated temperatures. In comparison to pristine WO3, the hydrogen-treated WO3 nanoflakes show an order of magnitude enhanced photocurrent, and more importantly, exhibit extraordinary stability for water oxidation without loss of photoactivity for at least seven hours. The enhanced photostability is attributed to the formation of substoichiometric WO3−x after hydrogen treatment, which is highly resistive to the re-oxidation and peroxo-species induced dissolution. This work constitutes the first example where WO3 can be stabilized for water oxidation in neutral medium without the need for oxygen evolution catalysts. The demonstration of electrochemically stable WO3 could open up new opportunities for WO3 based photoelectrochemical and photocatalytic applications.