This letter describes an aqueous-phase synthetic route to hydrous ruthenium oxide (RuO2) nanoparticles and their conversion into crystalline ones via a thermal annealing process. Electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were employed to characterize hydrous and crystalline RuO2 nanoparticles. The hydrous RuO2 nanoparticles exhibited higher activity for hydrogen evolution reaction than commercial Pt catalyst, while the crystalline RuO2 nanoparticles showed better performance for oxygen evolution reaction than IrO2 catalyst. With these hydrous and crystalline RuO2 catalysts, we were able to achieve highly efficient overall electrochemical water splitting. (C) 2017 Elsevier B.V. All rights reserved.