Co-catalysts have been widely studied to improve photocatalytic water oxidation efficiency, and the precise control of their oxidation states and minimizing particle sizes can optimize the practical performance. In this work, ultrafine cobalt oxide (CoOx) co-catalysts with specific Co valence states have been successfully synthesized, which are anchored on a tantalum oxynitride (TaON) host photocatalystviaa photochemical metal-organic deposition method. Photoelectrochemical and photoluminescence analyses prove that the Co2+dominant CoOxco-catalyst is favorable for charge separation and transportation because of its excellent hole-capturing properties, consequently enhancing the photocatalytic water oxidation performance. The resultant TaON with the Co2+dominant CoOxco-catalyst exhibits an outstanding O2production rate up to 6.10 ± 0.17 mmol h−1g−1and possesses an excellent apparent quantum efficiency of 21.2% at 420 nm, which are among the best values of water-oxidation photocatalysts.