The title salt, C(15)H(18)NO(2)(+)·Br(-)·H(2)O, is an analogue of the antidepressant drug agomelatine. The cation is protonated at the carbonyl O atom of its amide group. The side chain at the 1-position adopts an extended conformation, with all non-H atoms lying in the same plane as the naphthalene ring. This is in contrast with the crystal structures known for three agomelatine polymorphs, and also with two known cocrystals containing agomelatine. The structure displays three types of hydrogen bond, namely C=O-H···O, N-H···Br and O-H···Br, which define a two-dimensional network parallel to the (100) plane. The naphthalene rings interdigitate in a `zipper-like' fashion between these hydrogen-bonded networks, forming an offset arrangement. Direct face-to-face π-π contacts between naphthalene rings are not present in the structure.