Atomically two-dimensional semiconductor nanomaterials have drawn tremendous attention in photocatalytic applications due to their unique and remarkable properties. Herein, the assembly of TiO2 ultrathin nanosheets was fabricated from the titanate sheets using a gas-assisted liquid exfoliation method combined with hydrogenation treatment strategy. The hydrogenation treatment can introduce more surface point defects such as oxygen vacancies without damaging the ultrathin two-dimensional structure. The ultrathin two-dimensional assembly has high percentages of low-coordinated surface atoms and large specific surface areas (340 m(2)/g), which could increase the absorption of photon and accelerate the photocatalytic reaction process. Meanwhile, homogeneous oxygen vacancies and large fraction of low-coordinated surface atoms in TiO2 nanosheets can cause surface lattice distortion, which leads to a reduction in band gap and an upshift of conduction band minimum. The assembly possesses strong solar-light absorption, efficient charge transfer, and more surface reactive sites for photocatalytic reactions. As a result, the assembly exhibits fast photocatalytic hydrogen evolution rate of 540.7 mu mol h(-1) (30 mg catalyst) and good cycling stability under simulated solar irradiation. This work may provide perspectives for designing two-dimensional semiconductor materials besides TiO2 ultrathin nanosheets as photocatalysts.