The title compound, C 14H 11N 3O 3, adopts an E or trans configuration with respect to the C=N bond. In the molecule there is an intramolecular O—H⋯N hydrogen bond involving the hydroxy substituent at the 2-positon of the naphthalene ring and the adjacent methyleneamino N atom. The molecule is roughly planar, the dihedral angle between the naphthalene and imidazolidine-2,4-dione mean planes being 8.4 (1)°. In the crystal, pairs of N—H⋯O hydrogen bonds link molecules into inversion dimers. These dimers are futher linked via C—H⋯O interactions, forming a three-dimensional network.