A new monoclinic polymorphic form of the title compound, [Cu(HCO(2))(2)(C(14)H(12)N(2))(H(2)O)], is described. It differs from the first ortho-rhom-bic polymorph [Pan, Lin & Zheng (2005 ▶). Z. Kristallogr. New Cryst. Struct.220, 495-496] in the deviation of the Cu atom relative to the plane of the 2,9-dimethyl-1,10-phenanthroline (dmp) ligand. In the present structure, the Cu atom is shifted from the mean plane of the dmp ligand by only 0.005 (1) Å, compared with 0.318 (6) Å in the ortho-rhom-bic form. Hydrogen-bonding and π-π stacking inter-actions (mean inter-planar distance of 3.59 Å in the title compound) in the two different polymorphs are both essential to the supra-molecular assembly.