The title compound, C5H9NO4·H2O, is an isomer of the α-amino acid glutamic acid that crystallizes from water in its zwitterionic form as a monohydrate. It is not one of the 20 proteinogenic α-amino acids that are used in living systems and differs from the natural amino acids in that it has an α-methyl group rather than an α-H atom. In the crystal, an O-H⋯O hydrogen bond is present between the acid and water mol-ecules while extensive N-H⋯O and O-H⋯O hydrogen bonds link the components into a three-dimensional array.