Magnesium Oxide (MnO₂) nanoparticles were synthesized by Hydrothermal method. The synthesized samples were characterized using FT-IR, X-ray diffraction, AFM, and SEM. The adsorption of Orange G dye from aqueous solution onto MnO₂ as an adsorbent by batch method, under various experimental conditions including exposure time, MnO₂ dosage, pH, temperature, and initial Orange G dye concentration was investigated. Adsorption isotherms were used to test the adsorption data for Langmuir, Freundlich, Dubinin and Temkin as it was fit to Langmuir isotherm and type S according to Gilles’ classification. Thermodynamic functions’ data such as ΔH°, ΔG°, and ΔS° of the adsorption process were calculated, which showed that the adsorption endothermic process and the value of ΔG° was negative. This indicate that the adsorption happened spontaneously. The value of ΔS° is positive, which means the movement of dye molecules is unrestricted. Kinetic data were fit to pseudo-second order model.