The present work focuses on the effect of Cu substitution on the crystal structure and photocatalytic activity of nano-spinel oxides Ni(1−x)CuxAl2O4 (x = 0.0–1.0). The synthesized compounds by co-precipitation route are characterized by X-ray diffraction, FT-IR, X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy and UV–vis diffuse reflectance. The photocatalytic activity is followed by UV–vis spectroscopy and Electrochemical Impedance Spectroscopy in order to confirm the good performance of the catalyst and the charge separation of photogenerated (e−/h+) pairs. The photocatalytic efficiency of the synthesized catalysts is investigated through the decomposition of Congo Red dye under solar light irradiation. The efficient catalyst is Ni0.2Cu0.8Al2O4 with a removal conversion of 90.55% of the dye after 180 min. The parameters influencing the dye degradation like initial concentration are studied for the optimum degradation and the results have been discussed. This study shows that the adsorption kinetic of the Congo red has well followed the Langmuir isotherm model. The high photocatalytic activity of Ni0.2Cu0.8Al2O4 can be attributed to the valence band of the catalyst which enhances the mobility of the photoexcited charge carriers.