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Presence of phenazopyridine residues in water resources can be harmful to human body, hence it is very important to remove them from aqueous media. In this study, a magnetic nano-composite adsorbent, i.e. reduced graphene–iron oxide (rGO–Fe₃O₄) was synthesized and used for removal of phenazopyridine residues from wastewater samples. Experimental parameters influencing adsorption process were optimized, and results showed a sorption capacity of 14.064 mg g⁻¹ at pH=6.0 for phenazopyridine. Under the best experimental condition, rGO–Fe₃O₄ nanocomposite showed an efficiency of 91.4% for removal of phenazopyridine in aqueous solution. Adsorption of phenazopyridine on flat and parallel oxygen and iron–terminated rGO–Fe₃O₄ surfaces was studied by density functional theory calculations. It was found that, in case of both oxygen and iron–terminated rGO–Fe₃O₄ surfaces, phenazopyridine …