Nanotechnology is the newest and one of the most promising areas of research in modern medical sciences. Metal nanoparticles possess extraordinary optical, thermal, chemical properties and are being widely used in industrial, electronics, and biomedical fields. It has presented its potential to contribute to solving one of the greatest problems of the worldwide wastewater treatment issue. The approach of green synthesis of metal oxide nanoparticles seems to be a cost-efficient, eco-friendly, and easy alternative approach. The current study deals with the synthesis and characterization of Magnesium Oxide nanoparticles (MgO-NPs) using leaves extract of Iresine herbsti. The characterization was done by XRD and SEM. Then MgO-NPs were applied for the remediation of Reactive Brown 9 dye following the optimization of reaction parameters (conc. of dye, conc. of nanoparticles, pH, and temperature). The maximum decolorization (95.8%) was obtained at 0.02% dye conc., 0.003 mg/L conc. of MgO-NPs, at pH 4, and temperature 40 °C. TOC and COD were used for mineralization assessment of studied dye samples and their values were found to be 88.56% and 85.34%, respectively. The magnesium oxide nanoparticles could be applied successively for the treatment of other problematic dyes as well.