In the title compound, [Ni(C12H6O4)(C21H18N4)(H2O)] n , the Ni(II) cation is coordinated by three carboxyl-ate O atoms of two naphthalene-1,4-di-carboxyl-ate anions, one water mol-ecule and two N atoms of two 1,1'-(9,9-dimethyl-9H-fluoren-2,7-di-yl)di-1H-imidazole (DFDI) ligands, giving rise to a slightly distorted octa-hedral geometry. The Ni(II) ions are linked by the DFDI ligands into chains, which are further connected by the carboxyl-ate anions into double chains that elongate in the the b-axis direction. These double chains are linked by centrosymmetric pairs of O-H⋯O hydrogen bonds into layers parallel to (10-1). The asymmetric unit consists of one crystallographically independent Ni(II) cation, one carboxyl-ate and one DFDI ligand, as well as of one water mol-ecule, all of them located in general positions.