Single-crystal X-ray diffraction of Cs3ScSi6O15 shows the presence of main reflections and satellite reflections up to the fourth order along the c* direction. The (3+1)-dimensional incommensurately modulated structure was solved in superspace group X3m1(00gamma)0s0 [a = 13.861 (1), c = 6.992 (1) Å, V = 1163.4 (2) Å(3)] with a modulation wavevector q = 0.14153 (2)c*. Refinement of three modulation waves for positional and anisotropic displacement parameter values for all atoms converged to R(obs) values for all, main and satellite reflections of first, second and third order of 0.0200, 0.0166, 0.0181, 0.0214 and 0.0303, respectively. Cs3ScSi6O15 forms a mixed tetrahedral-octahedral framework with prominent six-membered rings of [SiO4]-tetrahedra interconnected by [ScO6]-octahedra. Apart from Sc, all atoms are strongly affected by positional modulation with maximum atomic displacements of up to 0.93 Å causing rigid polyhedral arrangements to perform tilt and twist movements relative to each other, such as a rotation of the Sc-octahedra around the 3-axis by over 38°. Cs has an irregular coordination environment; however, considering distances up to 3.5 Å, the bond-valence sum changes by no more than 0.02 as a function of t and thus overall kept at a level of ca 1.075.