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In this work, transition metal oxide clusters (MnO x , FeO x , CoO x , NiO x , and CuO x , denoted as TM-TiO 2 ) are in situ loaded on TiO 2 nanosheets through one-pot reaction. Structural and pore structural characterizations prove that metal ions do not dope into the frameworks of TiO 2 nanosheets. Through TEM and STEM, we can determine that clusters with ∼2 nm size are finely dispersed on TiO 2 nanosheets. PL spectra and photoelectrochemical measurements suggest that these metal oxide clusters can serve as hole traps. Time-resolved PL spectra demonstrate that the charge-transfer process in TM-TiO 2 is significantly accelerated, leading to higher charge separation efficiency. Metal oxide clusters show significant promotion effect in photocatalytic water oxidation to O 2 compared to RuO 2 /TiO 2 and IrO 2 /TiO 2 nanosheets (denoted as Ru-TiO 2 (IM) and Ir-TiO 2 (IM)) prepared through conventional impregnation method. We also prepared RuO 2 /TiO 2 and Ir/TiO 2 nanosheets (denoted as Ru-TiO 2 (HT) and Ir-TiO 2 (HT)) through the in situ loading method. Ru-TiO 2 (HT) and Ir-TiO 2 (HT) show O 2 evolution rates much better than those of Ru-TiO 2 (IM) and Ir-TiO 2 (IM) due to the smaller sizes of RuO 2 and IrO 2 . However, Mn-TiO 2 and Co-TiO 2 still display better photoactivities compared to those of Ru-TiO 2 (HT) and Ir-TiO 2 (HT). These results indicate that transition metal oxides with small sizes can also work as co-catalysts in photocatalysis to substitute noble metal oxides.