The title compound, [Cu(C(9)H(5)N(2)O(3))(2)(C(2)H(6)OS)(2)], consists of octahedrally coordinated Cu(II) ions, with the 3-oxo-3,4-dihydroquinoxaline-2-carboxylate ligands acting in a bidentate manner [Cu-O = 1.9116 (14) A and Cu-N = 2.1191 (16) A] and a dimethyl sulfoxide (DMSO) molecule coordinated axially via the O atom [Cu-O = 2.336 (5) and 2.418 (7) A for the major and minor disorder components, respectively]. The whole DMSO molecule exhibits positional disorder [0.62 (1):0.38 (1)]. The octahedron around the Cu(II) atom, which lies on an inversion centre, is elongated in the axial direction, exhibiting a Jahn-Teller effect. The ligand exhibits tautomerization by H-atom transfer from the hydroxyl group at position 3 to the N atom at position 4 of the quinoxaline ring of the ligand. The complex molecules are linked through an intermolecular N-H...O hydrogen bond [N...O = 2.838 (2) A] formed between the quinoxaline NH group and a carboxylate O atom, and by a weak intermolecular C-H...O hydrogen bond [3.392 (11) A] formed between a carboxylate O atom and a methyl C atom of the DMSO ligand. There is a weak intramolecular C-H...O hydrogen bond [3.065 (3) A] formed between a benzene CH group and a carboxylate O atom.