CoS nanoparticles and CoS/reduced graphene oxide (CoS/rGO) nanohybrids were fabricated by a unique single-mode microwave-assisted hydrothermal method. The microwave absorption properties of CoS/rGO composites with different rGO proportions were investigated. Morphology analysis indicated that the CoS nanoparticles were uniformly embedded into rGO without aggregation. The complex permittivity of CoS/rGO nanohybrids could be artificially tuned with the rGO proportions. For the CoS/rGO 1:2 composite, the minimum reflection loss (RL) of -56.9 dB was achieved at 10.9 GHz for the thickness of 2.2 mm; meanwhile, the RL exceeding -10 dB were obtained in the frequency range of 9.1-13.2 GHz. Compared with other rGO-based materials, CoS/rGO composite exhibited superior microwave absorption ability at a rather thin thickness and it has great potential to be used as a high-efficiency and tunable microwave absorber.