Photon upconversion from visible light to ultraviolet (UV) light is useful for various photochemical reactions such as artificial photosynthesis, but its efficiency is low under practical film conditions. Here, we demonstrate a film with a record-high visible-to-UV upconversion efficiency of 27.6% by simply soaking a porous film with a low-volatile upconversion solution. Furthermore, by combining this film with a microlens array, a significantly low threshold excitation light intensity of less than 0.60 mW cm⁻², at least one order of magnitude lower than solar irradiance, is achieved.