The aim of this work is to deposit cost-effective ZnO/g-C3N4 composite thin film on glass substrates and to study its photocatalytic dye degradation efficiency against three toxic organic dyes MB, MG and RhB. A simple and inexpensive nebulizer spray technique was employed to deposit the composite thin film samples. The photocatalysis experiment carried out against these three dyes showed that the addition of g-C3N4 with ZnO remarkably enhances the photocatalytic degradation efficiency of ZnO films. The degradation efficiency of ZnO against MB, MG and RhB is 74, 82 and 61%, respectively. The efficiency increases to 95, 97 and 73% after the addition of g-C3N4. The mechanism related to this enhancement is addressed with the help of optical and morphological results. Density Functional Theory calculations were carried out to validate the observed experimental results and support the proposed mechanism. (C) 2017 Elsevier B.V. All rights reserved.