Photocatalytic DNA damage by titanium dioxide was enhanced by the addition of glucose and galactose. Furthermore, the oxidized products of sugars by titanium dioxide photocatalysis induced DNA damage in the presence of copper(II) ion. This DNA damage was inhibited by catalase, indicating the contribution of hydrogen peroxide. The hydrogen peroxide generation from the oxidized sugars was confirmed by fluorometry. These findings suggest that the generation of secondary reactive oxygen species contributes to biomolecular damage by a titanium dioxide photocatalyst.