By two‐step sequential Pb2+ adsorption and reaction with methylammonium‐iodide (MAI) or ‐bromide (MABr) at a low concentration level of 0.06–0.10 m over mesoporous TiO2 or ZrO2 film, a well‐defined nanoscale CH3NH3PbI3 (MAPbI3) photosensitizer or CH3NH3PbBr3 (MAPbBr3) light emitter could be prepared in situ, respectively in a reproducible and atom‐economical way. The as‐prepared nanoscale perovskites are compared with their thin film counterparts in terms of light absorption/emission, crystallinity, surface morphology, and energy‐conversion efficiency. The nanoscale perovskite‐decorated films display more transparency than the bulky film due to the much lower amount deposited, while blueshifted and overwhelmingly brighter photoluminescence is observed in the “nano” relative to the “bulk” due to quantum size confinement. Transmission electron microscopy images also clearly show that a few nanometer‐sized perovskite dots are deposited homogeneously over the surface of TiO2‐ or ZrO2‐particulate film in the course of the current preparative route. When the nano‐MAPbI3 is tested as a photosensitizer in a solid‐state dye‐sensitized solar cell configuration with a very thin (≈650 nm) TiO2 mesoporous film, it has a promising initial power conversion efficiency of 6.23%, which outperformed the result of 2.28% from a typical organic molecular dye coded as MK‐2.