Water contamination with synthetic dyes and metal ions is an escalating problem, despite the huge research efforts put in the field of water treatment. The paper reports on one-step synthesis of reduced graphene oxide-cobalt oxide nanoparticles (rGO-Co3O4) nanocomposite under mild conditions. The nanocomposite material has been characterized using various analytical techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, UV–visible spectrophotometry and Fourier-transform infrared (FTIR) spectroscopy. The catalytic properties of the nanocomposite were demonstrated for the sodium borohydride-induced 4-nitrophenol reduction to 4-aminophenol. A full reduction has been achieved within less than 1 min at room temperature. Further, the nanocomposite was successfully applied for Cr(VI) adsorption with a maximum adsorption capacity of 208.8 mg.g−1, which is much higher than that obtained using many other magnetic adsorbents. Finally, the nanocomposite was found to be very efficient for anionic and cationic dye adsorption with complete removal within less than 2 min. The performance of the rGO-Co3O4 nanocomposite is quite high as compared to other graphene-based adsorbents. The ferromagnetic properties of the composite allowed effective separation and recyclability of the material by simple application of an external magnet.