We succeed in preparation of anatase TiO₂ single crystals with marked photocatalytic activity via a facile and effective method. This TiO₂ is composed of TiO₂ ultrathin nanosheets (2 nm in thickness) with 95% of exposed {100} facet, which is considered to be the active facet for photocatalytic reaction. This percentage (95%) is the highest among previously reported {100} facet exposed anatase TiO₂. More importantly, due to this high ratio, our developed TiO₂ nanosheets showed marked photocatalytic activity, about 5 times higher activity in both H₂ evolution and CO₂ reduction than the reference sample, TiO₂ cuboids with 53% of exposed {100} facet. For the TiO₂ nanosheets, both the higher percentage of exposed {100} facets and larger surface area can offer more surface active sites in the photocatalytic reaction. On the other hand, the superior electronic band structure which results from the higher percentage of {100} facet is also beneficial for the higher activity. This study exemplifies that the facet engineering of semiconductors is one of the most effective strategies to achieve advanced properties over photofunctional materials for solar energy conversion.