An object of the present invention is to provide a method for producing a surface-modified rare earth metal-based sintered magnet having extremely excellent corrosion resistance even in an environment with fluctuating temperature and humidity and also having excellent magnetic characteristics. The method for producing a surface-modified rare earth metal-based sintered magnet of the present invention as a means for achieving the object is characterized by comprising a step of subjecting a rare earth metal-based sintered magnet to a heat treatment at 200° C. to 600° C. in an atmosphere having an oxygen partial pressure of 1×103 Pa to 1×105 Pa and a water vapor partial pressure of 45 Pa or less with the ratio between the oxygen partial pressure and the water vapor partial pressure (oxygen partial pressure/water vapor partial pressure) being 450 to 20000.