In the structure of the title compound, C(18)H(15)ClN(4)O(4)S, the dihedral angle between the two benzene rings is 1.4 (3)°. The angle between the phenyl ring and thia-diazole ring is 5.8 (4)°. The conformations of the N-H and C=O bonds are anti with respect to each other. In the crystal structure, mol-ecules are linked by inter-molecular O-H⋯N, N-H⋯O and O-H⋯O hydrogen bonds, forming a three-dimensional network.