This paper describes a facile, single-step hydrothermal method to prepare ruthenium sulfide/thermally reduced graphene oxide (RuS2/TRGO) nanocomposites. In this synthesis procedure, aqueous solutions of RuCl3, l-cysteine, and graphene oxide are employed as the metal, sulfur, and graphene sources, respectively. The chemical structures and morphologies of the nanocomposites are characterized by X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Cyclic voltammetry, galvanostatic charge-discharge cycling, and electrochemical impedance spectroscopy are used to examine their electrochemical performances. The RuS2 nanoparticles (∼10 nm) uniformly disperse on the surfaces of the TRGO layers to form the RuS2/TRGO composite, which adequately inhibits aggregation of the RuS2 to fully exploit its impressive electrochemical activity and capacitance as a pseudocapacitive electrode material. The combination of the TRGO interconnected conductive networks and uniformly anchored RuS2 generates a specific capacitance of 193 F g−1 at a 5 mV s−1 scan rate, 150 F g−1 at a 0.5 A g−1 current density, good rate capability (57.3% retention at 6.25 A g−1), and reasonable cycle stability (90% retention of capacitance over 2000 cycles at a current density of 0.75 A g−1). Further, the RuS2/TRGO-30 composite electrode achieves energy densities of 20.84 and 6.11 Wh kg−1 at power densities of 250 and 3666.7 W kg−1, respectively. The RuS2/TRGO composites are promising for high-level energy storage applications because of their superior electrochemical activities.