There are three independent 3-hy-droxy-imino-1-methyl-indolin-2-one mol-ecules and a water mol-ecule in the asymmetric unit of the title compound, 3C(9)H(8)N(2)O(2)·H(2)O. The crystal packing is stablized by O-H⋯O and O-H⋯Nhydrogen bonds between 3-hy-droxy-imino-1-methyl-indolin-2-one mol-ecules and the water mol-ecule and weak π-π stacking inter-actions [centroid-centroid distances in the range 3.446 (2)-3.983 (2) Å], forming a two-dimensional network.