In this work, a novel method of preparing manganese oxide nanoparticles from an organometallic precursor dissolved in supercritical carbon dioxide (sc CO2) was presented. Using the new approach, nanomaterials mostly consisting of manganese oxides in beta-MnO2 and epsilon-MnO2 phases with small-sized (similar to 40 nm) grains and low polydispersity index (similar to 0.12) can be synthesized, which was consistently proved by means of thermogravimetric and X-ray diffraction analysis, scanning and transmission electron microscopy. Moreover, reasonable electrocatalytic activity of the obtained materials was detected by a rotating disk electrode method.