To provide: a method for polishing a diamond crystal such that the method eliminates affected regions including dislocations from the main surface of the diamond crystal; and a diamond crystal with the affected regions removed from the main surface. A diamond crystal having a main surface having a plane orientation of (100) is prepared. Next, mechanical polishing is performed on the main surface of the diamond crystal. A tangent contacts a curve that extends in a rotation direction in which the polishing wheel is rotating and that contacts a contact position at which the diamond crystal contacts the polishing wheel that is rotating. A tangent direction of the tangent at the contact position is within a range of ±10° (degrees) relative to a direction of the diamond crystal. By this mechanical polishing, an affected region is caused to appear such that the affected region is parallel to a direction of a plane orientation (111) of the diamond crystal and penetrates the diamond crystal onto the main surface. Next, chemical mechanical polishing is performed on the main surface of the diamond crystal to remove the affected region parallel to the direction of the plane orientation (111), thereby removing the affected region from the main surface.