The crystal structure of trisodium uranate, which forms following the interaction between sodium and hyperstoichiometric urania, has been solved for the first time using powder X-ray and neutron diffraction, X-ray absorption near-edge structure spectroscopy, and solid-state Na-23 multiquantum magic angle spinning nuclear magnetic resonance. The compound, isostructural with Na3BiO4, has monoclinic symmetry, in space group P2/c. Moreover, it has been shown that this structure can accommodate some cationic disorder, with up to 16(2)% sodium on the uranium site, corresponding to the composition alpha-Na-3(U1-x,Na-x)O-4 (0 < x < 0.18). The a phase adopts a mixed valence state with the presence of U(V) and U(VI). The two polymorphs of this compound described in the literature, m- and beta-Na-3(U1-x,Na-x)O-4, have also been investigated, and their relationship to the alpha phase has been established. The completely disordered low-temperature cubic phase corresponds to a metastable phase. The semiordered high-temperature beta phase is cubic, in space group Fd (3) over barm.