The crystal structure of the title compound, [Cu(C(4)H(10)N(5))(2)].H(2)O, contains two independent copper N,N-dimethylbiguanide complex units, each with square-planar coordination of the Cu atom by four N atoms. The two complexes have different symmetry, with one Cu atom lying on an inversion centre and the other on a twofold rotation axis. The Cu-N bond lengths are 1.923 (2) and 1.950 (2) A in the centrosymmetric complex, and 1.928 (2) and 1.938 (2) A in the non-centrosymmetric complex. The crystal structure is stabilized by N-H...O, O-H...N and N-H...N hydrogen bonds; each water molecule forms four hydrogen bonds involving three different Cu complexes.