Cocrystallization of melamine (1,3,5-triazine-2,4,6-triamine, ma) with (2-carboxyethyl)(phenyl)phosphinic acid (H(2)L) from water affords the title compound, C(3)H(7)N(6)(+)·C(9)H(10)O(4)P(-)·H(2)O or (maH)(HL)·H(2)O, (I). The phosphinic acid H atom of each H(2)L molecule is transferred to a melamine molecule. Structural analysis reveals that there are two types of secondary building units in the crystal structure, namely cationic [(maH(+))(2)](∞) ribbons and anionic {[(HL)(2)(H(2)O)(2)](2-)}(∞) layers, the combination of which through hydrogen-bond and electrostatic interactions, generates a large-scale two-dimensional layered structure. The thick layer is sandwich-like, with the central [(maH(+))(2)](∞) ribbons being further stabilized by π-π stacking interactions. It is also worthy of note that two conformational isomeric R(6)(5)(24) hydrogen-bond ring motifs can be identified in the {[(HL)(2)(H(2)O)(2)](2-)}(∞) layer.