The structure of Sm 2/3 Cr 2 S 4 has been solved from single-crystal X-ray diffraction data. This compound presents a 3 ×asuperstructure alongaand an incommensurate superstructure alongbclose to 3 ×b. The basic structure is described in the space groupPmnbwitha= 3.5030(3) Å,b= 10.9199(8) Å,c= 12.7987(8) Å,Z= 4,R= 0.046 for 916 observed reflections. The modulated structure refinement, taking into account the two superstructures, was carried out in a (3 + 2)D superspace group and leads toR= 0.063 for 1706 reflections. This structure consists of staggered double columns of [CrS 6 ] octahedra connected by samarium atoms in a trigonal prismatic bicapped environment. This structure is reminiscent of that found in the lillianite mineral family. The statistical occupancy of samarium atoms (1/3), in agreement with the charge equilibrium (Sm 3+ , Cr 3+ , S 2− ), is related to the superstructure.