A novel, facile, one-step Sn⁴⁺-based anodic deposition method was developed to flatten fluorine-doped tin oxide (FTO) layer, leading to a significant enhancement in visible light transmittances, from 79 to 85%. The Sn²⁺ necessary for the anodic deposition of SnO₂ was generated in situ from the starting Sn⁴⁺ such that a slow and balanced deposition and acid-etching of SnO₂ became possible. Furthermore, the fluoride ions released from the acid-etching of the starting FTO layer were later incorporated into the newly deposited SnO₂ to make it fluorine-doped, giving rise to the formation of the conductive, much flattened FTO layer. The flattening of the FTO layer led to a significant light scattering reduction at the FTO-air interface and thus enhanced the light transmittances.