A method for producing a sintered R-T-B based magnet includes providing a sintered R-T-B based magnet body, where T is mostly Fe; providing an RH diffusion source that includes 0.2 mass % to 18 mass % of a light rare-earth element RL; 40 mass % to 70 mass % of Fe; and a heavy rare-earth element RH as the balance; and performing an RH diffusion process by loading the sintered R-T-B based magnet body, a stirring aid member, and the RH diffusion source into a chamber, and by heating the sintered R-T-B based magnet body, the stirring aid member, and the RH diffusion source to a temperature of 700° C. to 1000° C. while rotating or rocking the chamber. The Fe/RH ratio is within a range from two to seven and is defined by a mass fraction of Fe when a mass fraction of the heavy rare-earth element RH in the RH diffusion sources is three.