A reduced graphene oxide (rGO)/cobalt sulfide composite is synthesized with a simple and efficient ultrasound-assisted wet chemical method. The morphology and microstructure of the composite are examined with field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results confirm that cobalt sulfide nanoparticles are homogeneously and tightly attached on the surfaces of rGO. As an anode material for lithium-ion batteries, this composite delivers a high reversible capacity of 994 mA h g−1 after 150 cycles at a current density of 200 mA g−1. A synergistic effect combining the merits of rGO and cobalt sulfide nanoparticles endows the composite with superior electrochemical performances over those of pure cobalt sulfide.