We report a new method for the determination of photothermal conversion efficiency of photothermal agents, based on the use of an integrating sphere. We validated this method by comparing the photothermal conversion efficiency of Au nanorods calculated by this method and by the more conventional time constant method. Then, we applied this method to determine the photothermal conversion efficiency of graphene and graphene oxide nanosheets dispersions in dimethylformamide and water, respectively, finding out that they are excellent photothermal agents with photothermal conversion efficiencies among the highest reported up to now. We also analyzed the influence of the concentration of the materials, and the wavelength and power of irradiation in the temperature increase that can be achieved with them, finding out that they can be used, for instance, in cancer treatment through hyperthermia procedures with reduced costs when compared to other photothermal agents.