Titania, which exhibits superwetting under light illumination, has been widely used as an ideal material for environmental solution such as self-cleaning, water–air purification, and antifogging. There have been various studies to understand such superhydrophilic conversion. The origin of superwetting has not been clarified in a unified mechanism yet, which requires direct experimental investigation of the dynamic processes of water-layer growth. We report in situ measurements of the growth rate and height of the photo-adsorbed water layers by tip-based dynamic force microscopy. For nanocrystalline anatase and rutile TiO₂ we observe light-induced enhancement of the rate and height, which decrease after O₂ annealing. The results lead us to confirm that the long-range attraction between water molecules and TiO₂, which is mediated by delocalized electrons in the shallow traps associated with O₂ vacancies, produces photo-adsorption of water on the surface. In addition, molecular dynamics simulations clearly show that such photo-adsorbed water is critical to the zero contact angle of a water droplet spreading on it. Therefore, we conclude that this “water wets water” mechanism acting on the photo-adsorbed water layers is responsible for the light-induced superwetting of TiO₂. Similar mechanism may be applied for better understanding of the hydrophilic conversion of doped TiO₂ or other photo-catalytic oxides.