Summary: Ferrinatrite crystallizes in space group P3¯, with a = 15·566(5), c = 8·69(1) Å, and Z = 6. The crystal structure was solved by three-dimensional Patterson and Fourier syntheses, and refined by least squares employing 2378 independent reflexions to a final R value of 0·068. The iron ions occupy special positions and are surrounded octahedrally by oxygen atoms. Fe³⁺O₆ octahedra and SO₄, tetrahedra are linked together to form infinite chains of Fe-O-S linkages in the [0001] direction. These chains are linked to each other by [NaO₅(H₂O)₂] polyhedra and probably by hydrogen bonds. The topology of the arrangement is the same as that of the hypothetical P₃₁₂ structure proposed by Moore and Araki (1974).