In the title isoniazid derivative, C(13)H(11)N(3)O(4)·2H(2)O, the Schiff base mol-ecule exists in an E configuration with respect to the acyclic C=N bond. An intra-molecular O-H⋯N hydrogen bond forms a six-membered ring, producing an S(6) ring motif. The essentially planar pyridine ring [maximum deviation = 0.0119 (8) Å] is inclined at a dihedral angle of 7.30 (4)° with respect to the benzene ring. In the crystal, inter-molecular O-H⋯N, O-H⋯O, N-H⋯O and C-H⋯O hydrogen bonds link the mol-ecules into two-dimensional arrays lying parallel to the (10) plane. These arrays are further inter-connected into a three-dimensional extended network via O-H⋯O and C-H⋯O hydrogen bonds. A weak inter-molecular π-π inter-action [centroid-to-centroid distance = 3.5627 (5) Å] is also observed.