The title compound, sodium dioxidomolybdenum(VI) arsenate(V), β-NaMoO(2)AsO(4), was prepared by solid-state reaction at 953 K. In the crystal structure, the AsO(4) tetra-hedra and MoO(6) octa-hedra (both with m symmetry) share corner atoms to form a three-dimensional framework that delimits cavities parallel to [010] where disordered six-coordinated sodium cations (half-occupation) are located. Structural relationships between the different orthoarsenates of the AMoO(2)AsO(4) series (A = Ag, Li, Na, K and Rb) are discussed.