Approximately 15 nm thick nitrogen-doped lanthanum titanate (La₂Ti₂O₇) nanosheets with a single-crystalline perovskite structure have been prepared by hydrothermal processing and subsequent heat treatment in NH₃ at 600 ℃. Doping nitrogen into the La₂Ti₂O₇ nanosheets results in the narrowing of the band gap, extending the light absorption into the visible light region (~495 nm). The nitrogen-doped La₂Ti₂O₇ nanosheets not only show significant visible light photocatalytic activity toward the decomposition of methyl orange but also exhibit enhanced the ultraviolet light photocatalytic activity. The enhancement of photocatalytic activity originates from the narrowing of the band gap of La₂Ti₂O₇ nanosheets. The results obtained show that the desirable route to extend the photocatalytic activity of a semiconductor from the ultraviolet to the visible light region is to narrow the band gap rather than to create localized mid-gap states.