The removal of textile azo dye, Reactive Black from the aqueous solutions by catalytic wet air oxidation in the presence of LaNiO3 perovskite catalyst has been investigated. The most suitable reaction conditions were determined by testing various the catalyst loadings, reaction temperature and pressure values, and the initial pH of the Reactive Black 5 solutions. The most suitable reaction conditions with 0.61 L/min of air flow rate were found to be 1 g/L of LaNiO3 loading, 50 °C of reaction temperature, 1 atm of reaction pressure, and, pH = 3 for the oxidation of 100 mg/L Reactive Black solutions. Under these conditions the degradation and the decolorization efficiencies were evaluated as 65.4% and 89.6%, respectively. The phytotoxicity analyzes were carried out by using Lepidium sativum. According to the toxicity tests a remarkable decrease in the growth inhibition was achieved by the catalytic wet air oxidation in the presence of LaNiO3 catalyst. The growth inhibition in the untreated and treated dye solutions were calculated as 49.3% and 23.7%, respectively.