Although Al2O3 is considered photocatalytically inactive, in this research, novel Al-rich Al2O3 ultra-thin films are employed for the first time as fast and active photocatalysts for water treatment from organic dyes. These films are prepared using plasma enhanced atomic layer deposition (ALD). Structure of the films is investigated using X-ray diffraction, X-ray photoelectron spectroscopy and high resolution scanning electron microscope showing amorphous structure. Films with different thicknesses (8–38 nm) are applied for photo-degradation of three dyes (Levafix Brilliant Red E−4BA, C.I. Reactive Orange 96 and Remazol Black 5) commonly used in the textile industry causing severe problems in the textile wastewater. As a further novelty, the thinner film (8 nm) is surprisingly about five times faster to decompose the dyes than the thicker film (38 nm). Enhancing the photocatalytic activity could be attributed to decreasing the energy gap from 4.75 to 4.3 eV with increasing the Al composition associated with decreasing the thickness. Also, existence of excess Oxygen vacancies in the amorphous Al2O3 provides the band gap with more defect levels which reduce recombination of the photo-generated charge carriers and increase their life time. Thus, they can reach surface of the film and participate in the photocatalysis process. This indicates the promising usage of Al-rich Al2O3 ultra-thin films in water treatment to save the time and cost significantly.