The title monohydrate, C9H10N2O3·H2O, contains an aziridine ring including two contiguous stereocenters, both of which exhibit an R configuration. The methylhydroxy and nitrophenyl groups are cis-disposed about the aziridine ring. The mean plane of the benzene ring is tilted to the aziridine ring by 66.65 (8)°. The nitro group is nearly coplanar with the benzene ring [dihedral angle = 2.5 (2)°]. In the crystal, the components are linked by N—H⋯O, O—H⋯N and O—H⋯O hydrogen bonds, generating supramolecular layers parallel to (001).