A method for producing a sintered R-T-B based magnet includes providing a sintered R-T-B based magnet body, of which the rare-earth-element mole fraction falls within the range of 31 mass % to 37 mass %; providing an RH diffusion source including a heavy rare-earth element RH (which is at least one of Dy and Tb) and 30 mass % to 80 mass % of Fe; loading the sintered magnet body and the RH diffusion source into a processing chamber so that the magnet body and the diffusion source are movable relative to each other and readily brought close to, or in contact with, each other; and performing an RH diffusion process by conducting a heat treatment on the sintered magnet body and the RH diffusion source at a process temperature of 700° C. to 1000° C. while moving the sintered magnet body and the RH diffusion source either continuously or discontinuously.