We report a spontaneous phase transformation of titania nanotubes induced by water at room temperature, which enables the as-anodized amorphous nanotubes to be cystallized into anatase mesoporous nanowires without any other post-treatments. These mesoporous TiO₂ nanomaterials have a markedly improved surface area, about 5.5 times than that of the as-anodized TiO₂ nanotubes, resulting in a pronounced enhanced photocatalytic activity. The present approach not only allows a flexible control over the morphology of TiO₂ nanostructures but can fundamentally eliminate the need for high temperature operations for crystallizing amorphous TiO₂.