In the title compound, (C4H12N2)[CdCl4]·H2O, the [CdCl4]2− anions adopt a slightly distorted tetrahedral configuration. In the crystal, O—H⋯Cl hydrogen bonds link the anions and water molecules into corrugated inorganic chains along the b axis which are interconnected via piperazinediiumN—H⋯O and N—H⋯Cl interactions into a three-dimensional framework structure