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The facile electron-hole pair recombination in earth-abundant transition-metal oxides is a major limitation for the development of highly efficient hydrogen evolution photocatalysts. In this work, the thickness of a layered β-CoOOH semiconductor that contains metal/hydroxy groups was reduced to obtain an atomically thin, two-dimensional nanostructure. Analysis by ultrafast transient absorption spectroscopy revealed that electron-hole recombination is almost suppressed in the as-prepared 1.3 nm thick β-CoOOH nanosheet, which leads to prominent electron-hole separation efficiencies of 60-90 % upon irradiation at 350-450 nm, which are ten times higher than those of the bulk counterpart. X-ray absorption spectroscopy and first-principles calculations demonstrate that [HO−CoO6− x] species on the nanosheet surface promote H+ adsorption and H2 desorption. An aqueous suspension of the β-CoOOH nanosheets exhibited a high hydrogen production rate of 160 μmol g−1 h−1 even when the system was operated for hundreds of hours. [ABSTRACT FROM AUTHOR]