A hair treatment agent in which penetrability of a modified peptide into the internal structure of the hair can be enhanced even under the conditions of acidic pH, and a method for causing a modified peptide to penetrate into the hair, are provided.Disclosed is a hair treatment agent at a pH of from 3.5 to 5.5, the hair treatment agent including: (A) a modified peptide having one kind or two or more kinds of side chain groups selected from a side chain having a structure represented by the following Formula (I), a side chain group having a salt of a structure represented by the following Formula (I), a side chain group having a structure represented by the following Formula (II), a side chain group having a salt of a structure represented by the following Formula (II), a side chain group having a structure represented by the following Formula (III), and a side chain group having a salt of a structure represented by the following Formula (III) and (B) an anionic surfactant:—S—S—(CH2)n—COOH (I)(wherein in Formula (I), n represents 1 or 2)—S—S—CH(CH3)—COOH (II)—S—S—CH(COOH)—CH2—COOH (III).
