In the title compound, [CuCl2(C4H4N2O)(H2O)], the Cu-II cation is coordinated by two chloride anions, one pyrimidin-2-one N atom and one water molecule, giving a slightly distorted square-planar geometry. In the crystal structure, the pyrimidin-2-one rings stack along the b axis, with an interplanar distance of 3.306 angstrom, as do the copper coordination planes (interplanar spacing = 2.998 angstrom). The coordination around the Jahn-Teller-distorted CuII ion is completed by long Cu center dot center dot center dot O [3.014 (5) angstrom] and Cu center dot center dot center dot Cl [3.0194 (15) angstrom] interactions with adjacent molecules involved in this stacking. Several N H center dot center dot center dot Cl, O-H center dot center dot center dot Cl and O-H center dot center dot center dot O intermolecular hydrogen bonds form a polar three-dimensional network.