The title compound, C(8)H(7)NO(4)S·H(2)O, was obtained by reaction of 2-mercaptopyridine-3-carboxylic acid with chloro-acetic acid. In the mol-ecular structure, the dihedral angle between the two least-squares planes defined by the pyridine ring and the carb-oxy group is 8.32 (9)°. The carboxy-methyl-sulfanyl group makes a torsion angle of 82.64 (12)° with the pyridine ring. An intra-molecular O-H⋯N hydrogen bond between the acidic function of the carboxy-methyl-sulfanyl group and the pyridine N atom stabilizes the conformation, whereas inter-molecular O-H⋯O hydrogen bonding with the uncoordinated water mol-ecules is responsible for packing of the structure, leading to chains propagating in [001].