Direct current reactive sputtering deposition of ruthenium oxide thin films (bottom and top electrodes) at 400°C are performed to produce a solid-state thin-film supercapacitor (TFSC). The supercapacitor has a cell structure of RuO2/Li2.94PO2.37N0.75 (Lipon)/RuO2/Pt. Radio frequency, reactive sputtering deposition of an Li2.94PO2.37N0.75 electrolyte film is performed on the bottom RuO2 film at room temperature to separate the bottom and top RuO2 electrodes electrically. The stoichiometry of the RuO2 thin film is investigated by Rutherford back-scattering spectrometry (RBS). X-ray diffraction (XRD) shows that the as-deposited RuO2 thin film is an amorphous phase. Scanning electron microscopy (SEM) measurements reveal that the RuO2/Lipon/RuO2 hetero-interfaces have no inter-diffusion problems. Charge–discharge measurements with constant current at room temperature clearly reveal typical supercapacitor behaviour for a RuO2/Lipon/RuO2/Pt cell structure. Since the electrolyte thin film has low ionic mobility, the capacity and cycle performance are inferior to those of a bulk type of supercapacitor. These results indicate that a high performance, TFSC can be fabricated by a solid electrolyte thin film with high ionic conductivity.