The title compound, C(13)H(10)N(4)O(2)S, was prepared through a cyclization reaction of 1,10-phenanthroline-5,6-dione and thio-urea. The dihedral angle between the pyridine rings is 8.22 (2)°. In the crystal, mol-ecules are connected by N-H⋯O, O-H⋯N, N-H⋯S and O-H⋯S hydrogen bonds, forming a three-dimensional network.