The title compound, alternatively named sodium 6-nitro-3H-1,2,3-benzoxathiazole 2,2-dioxide monohydrate, Na+·C6H3N2O5S-·H2O, consists of chains of NaO7 units, with the seven donor-O atoms coming from two water molecules and five p-nitrobenzoxasulfamate anions. The seven-coordinate geometry around the Na+ ion is described as monocapped trigonal prismatic, but with a large distortion from ideal geometry. Each triangular face is defined by one O atom each from a water molecule, a nitro group and a sulfonyl group. An O atom from a sulfonyl group caps one of the square faces of the trigonal prism in an unsymmetrical fashion. The water molecules and one sulfonyl O atom are involved in bridging adjacent units, as is the nitro group of the anion. The sulfamate ions adopt an antiparallel alignment between the NaO7 units and are connected to each other by C-H⋯O and π-π interactions. The three-dimensional crystal structure is stabilized by a network of strong O-H⋯N hydrogen bonds.