A novel quaternary scandium borocarbosilicide Sc 3.67− x B 41.4− y− z C 0.67+ z Si 0.33− w was found. Single crystallites were obtained as an intergrowth phase in the float-zoned single crystal of Sc 0.83− x B 10.0− y C 0.17+ y Si 0.083− z that has a face-centered cubic crystal structure. Single crystal structure analysis revealed that the compound has a hexagonal structure with lattice constants a = b = 1.43055(8) nm and c = 2.37477(13) nm and space group P 6 ̄ m2 (No. 187). The crystal composition calculated from the structure analysis for the crystal with x = 0.52, y = 1.42, z = 1.17, and w = 0.02 was ScB 12.3C 0.58Si 0.10 and that agreed rather well with the composition of ScB 11.5C 0.61Si 0.04 measured by EPMA. In the crystal structure that is a new structure type of boron-rich borides, there are 79 structurally independent atomic sites, 69 boron and/or carbon sites, two silicon sites and eight scandium sites. Boron and carbon form seven structurally independent B 12 icosahedra, one B 9 polyhedron, one B 10 polyhedron, one irregularly shaped B 16 polyhedron in which only 10.7 boron atoms are available because of partial occupancies and 10 bridging sites. All polyhedron units and bridging site atoms interconnect each other forming a three-dimensional boron framework structure. Sc atoms reside in the open spaces in the boron framework structure.