Different phase nanostructured MnO₂, including α-, β-, γ-, and δ-MnO₂, were prepared by a hydrothermal route. Their photocatalytic activities in the degradation of cationic (methylene blue) and anionic (methyl orange) dyes were compared for the first time. The as-prepared oxides were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and low-temperature nitrogen adsorption-desorption techniques. It was shown that the highest efficiency was achieved in the degradation of cationic methylene in the presence of the δ-MnO₂ photocatalyst (conversion of methylene blue was 90.4% after 180 min). These results are explained by unique physicochemical properties of the δ-MnO₂ polymorph. It was observed that there is a synergetic effect between various physicochemical properties of δ-MnO₂ which allows to obtain the high photodegradation efficiency: high adsorption ability towards methylene blue, unique 2D morphology, lower Mn–O bond strength, and high surface Mn⁴⁺/Mn³⁺ molar ratio. The reaction mechanism as well as the main products of the methylene blue photooxidation process over δ-MnO₂ were also investigated.