Dendrimer-encapsulated ruthenium oxide nanoparticles (DEN-RuO2) have been used as catalysts in lithium-oxygen (Li-O-2) batteries for the first time. The results obtained from ultraviolet-visible spectroscopy, electron microscopy and X-ray photoelectron spectroscopy show that the nanoparticles synthesized by the dendrimer template method are ruthenium oxide, not metallic ruthenium as reported by other groups. The DEN-RuO2 significantly improves the cycling stability of Li-O-2 batteries with carbon electrodes and decreases the charging potential even at ten times less catalyst loading than those reported previously. The monodispersity, porosity, and large number of surface functionalities of the dendrimer template prevent the aggregation of the RuO2 nanoparticles, making their entire surface area available for catalysis. The potential of using DEN-RuO2 as a standalone cathode material for Li-O-2 batteries is also explored.