The bacterium Pseudomonas aeruginosa BCH decolorized and degraded the sulphonated azo dye Remazol Orange in plain distilled water. The effects of different parameters, i.e. pH, temperature and cell mass concentration on the biodegradation of dye in aqueous phase was evaluated using response surface methodology. Optimization was carried out using three-level Box–Behnken design. Predicted values were found to be in good agreement with experimental values (R 2 0.9997 and pred R 2 0.9984), which indicated suitability of the employed model and the success of response surface methodology. Optimum dye decolorization was maximized and the favourable conditions were pH 7.43, temperature 29.39 °C and cell mass concentration 2.88 g l−1, which resulted in 96.01 % decolorization within 5 h. It was validated from the predicted response (97.37 %). According to the analysis of variance results, the proposed model can be used to navigate the design space. 3D plot analysis disclosed the significant interaction between all three tested factors on decolorization process. The combinations of the three variables predicted during response surface methodology were confirmed through confirmatory experiments. Observations indicated that higher cell mass accelerated the decolorization process. Degradation of the dye was verified through high performance liquid chromatography analysis. Phytotoxicity studies carried out with dye and dye metabolites using Phaseolus mungo, Triticum aestivum and Sorghum vulgare indicated the detoxification of dye.