The invention relates to a cosmetic product refill (32) for a device for treating the hair that comprises at least one heating element (26; 28), the cosmetic product refill (32) comprising a means for fastening the refill (32) to the device and a cosmetic product applicator element in the form of a sheet, such that the means for fastening the refill is designed to fasten the cosmetic product refill (32) to the device for treating the hair in such a way that a part of the cosmetic product refill (32) is offset laterally with respect to the heating element(s) (26; 28), and such that at least one portion of the applicator element is designed to extend next to the heating element(s) (26; 28). The invention also relates to a device for treating the hair (10) including such a refill.