The crystal structure of darapskite, Na₃(NO₃)(SO₄)·H₂O, was determined and refined using three-dimensional X-ray data collected from a synthetic crystal. The crystals are monoclinic, space group P21/m, with a = 10.564, b = 6.911, c = 5.194 Å, $\beta$ = 102.78°. Two molecules are in the unit cell. Intensities were estimated by a microdensitometer from Weissenberg photographs about the b axis. The structure was derived from three-dimensional Patterson and electron-density maps and refined by the least-squares method. The final R index for 542 observed reflections is 0.09. In the structure the sodium atom Na₁ coordinates six oxygen atoms according to a distorted octahedron, whereas the sodium atom Na₂ links seven oxygen atoms building up a polyhedron similar to an irregular octahedron with a centered edge. The SO₄ and NO₃ groups are quite regular. The Na₁ octahedra are connected to each other alternatively by one face and one edge, to form a chain in the b direction. Among these chains are located the Na₂ polyhedra as well as the SO₄ and NO₃ groups, which form strong connections, mainly in the (010) plane. The crystals of darapskite show a reticular pseudo-merohedral twinning, the twin axis being [001], with an obliquity of 1° 26' and an index of 2.