Single crystals of the new borosilicate feldspar with nominal composition KBSi3O8 were grown hydrothermally. The crystal structure of KBSi3O8 was determined by direct methods: it is monoclinic, C2/m, a=8.4377(8), b=12.4152(11), c=6.8769(7) Å, β = 116.133(7)°, V = 646.75(11) Å3, dcalc = 2.693 g/cm3. The structure corresponds to that of monoclinic aluminosilicate feldspar with differences only in the size and density of the tetrahedral framework due to the presence of boron instead of aluminium. The distribution of Si and B over the tetrahedral sites is partially ordered with one Si-rich tetrahedron ( = 1.60 Å), and another one occupied by Si and B in almost equal proportion ( = 1.55 Å). According to the high temperature X-ray diffraction study the expansion of the structure is highly anisotropic: α1 = 21, α2 = 2, α3 = 5 × 10–6 K–1, ∠(α3c) = 7°. The direction of maximal expansion approximately coincides with the short diagonal of the ac parallelogram. KBSi3O8 obtained by hydrothermal synthesis is