The structure of the title compound, catena-poly[[[di-μ-aqua-μ-fumarato-μ-(boric acid)-disodium]-di-μ-aqua] boric acid monosolvate], contains two crystallographically independent Na(+) cations, each being six-coordinated by one fumarate O atom, one boric acid O atom and four water O atoms in a distorted octa-hedral geometry. Adjacent [NaO(2)(OH(2))(4)] units share edges and are linked into chains propagating parallel to [100]. The free boric acid mol-ecules are connected to the chains through strong inter-molecular O-H⋯O hydrogen bonds. Additional O-H⋯O hydrogen bonds between the water mol-ecules, the free and coordinated boric acid mol-ecules and the fumarate anion lead to the formation of a three-dimensional supra-molecular structure. With the exception of the two water mol-ecules, all other atoms lie on mirror planes.