Oxygen vacancies in catalyst systems play a crucial role in manipulating pivotal redox properties that are strongly dependent on the composition of the material. Herein, for the first time, experimental evidence of a linear correlation between the extent of oxygen vacancy formation in the La0.6Sr0.4BO3 (B=Cr, Mn, Fe, Co, and Ni) perovskite series and H2 generation in two‐step thermochemical water splitting is reported, with detailed materials characterization by means of thermogravimetric analysis, XRD, SEM, TEM, and energy‐dispersive X‐ray spectroscopy. Noteworthy O2 (718 μmol g−1) and H2 (514 μmol g−1) production was achieved by the La0.6Sr0.4CoO3 perovskite in the thermochemical water‐splitting process conducted between 1300 and 900 °C.