Improvement in a process for producing an arylazo-aminonaphtholsulfonic acid represented by a free acid of the formula, ##STR1## wherein A is a phenyl or naphthyl group unsubstituted or substituted by one to four members selected from alkyl, alkoxy, phenoxy, sulfonic acid, carboxylic acid, arylamino, amino, nitro and hydroxyl groups and halogen atoms, and m is 0 or 1, which comprises reacting an aminonaphtholsulfonic acid represented by a free acid of the formula, ##STR2## wherein m is as defined above, with a dibasic acid anhydride to obtain an acylaminonaphtholsulfonic acid, coupling a diazonium compound of amine of the formula,A'--NH.sub.2wherein A' is a phenyl or naphthyl group unsubstituted or substituted by one to four members selected from alkyl, alkoxy, phenoxy, sulfonic acid, carboxylic acid, arylamino, acrylamino, nitro and hydroxyl groups and halogen atoms, with the acylaminonaphtholsulfonic acid, and then subjecting the resulting coupling product to hydrolysis in the presence of an acid or an alkali. The arylazo-aminonaphtholsulfonic acid is useful as acid dyes or acid mordant dyes in itself as well as an important intermediate for the production of reactive dyes or direct dyes.