In the title hydrate, C(4)H(7)N(3)O(2)·H(2)O, all the non-H atoms lie on a crystallographic mirror plane. The H atoms of both methyl groups are disordered over two sets of sites. In the crystal, N-H⋯O(w) and O(w)-H⋯O(k) (w = water and k = ketone) hydrogen bonds link the components into (010) sheets.