In this research, manganese oxides (MnO₂) nanoparticles were prepared by hydrothermal method using KMnO₄ as a precursor. The final brown–black precipitate MnO₂ nanoparticles as prepared, and annealed at different temperatures (250, 450, and 750 °C) were characterized. The nanoparticles prepared were tested for removal of methylene blue (MB), used as a model dye from water. In order to determine the structure and the chemical nature of the MnO₂ nanoparticles prepared, the characterization was carried out by X-ray diffraction. For the surface morphological studies of nanoparticles, field emission scanning electron microscopy was used. In order to study the surface roughness atomic force microscopy was used for determination of the imaging surface structures in the nm scale. Fourier transform infrared spectrometry was used to investigate the vibrations of functional groups in MnO₂. The tests for dye removal from water using MnO₂ nanoparticles have been carried out for MnO₂ nanoparticles as prepared and annealed at different temperatures. The process parameters such as speed of shaking, reaction time, and MB concentration were studied at 25 °C temperature to determine the best removal efficiency of methylene blue from water. UV/Visible spectrophotometer was used to follow the MB removal. MnO₂ annealed at 750 °C exhibited the highest MB removal efficiency, 89%, as compared with MnO₂ nanoparticles as prepared and annealed at 250 and 450 °C.