Disclosed is crystalline agomelatine hydrochloride monohydrate as represented by general formula (I), wherein X is Cl. Further disclosed is a method for the preparation of agomelatine hydrochloride monohydrate, wherein agomelatine is reacted with hydrochloric acid in an aqueous organic solvent to produce the agomelatine hydrochloride hydrate. Also disclosed is a pharmaceutical composition, comprising an agomelatine hydrochloride hydrate as defined above in associated with one or more pharmaceutically acceptable adjuvants or excipients.
