A new compound, Rb₃B₇O₁₂, has been synthesized using solid state reaction and glass crystallization methods. It is triclinic, space group P-1, a = 6.603(1), b = 6.632(1), c= 30.085(6), Å, α = 91.183(4), β = 91.781(4), γ = 119.293(3)°, V = 1147.5(5) ų, Z =4. The crystal structure of the compound has been solved by direct methods and refined to R₁ = 0.068 (ωR = 0. 142) on the basis of 1840 unique observed reflections (|Fo| > 4σ |Fo|). The structure is based upon a novel [B₇O₁₂] layered anion. The layer is parallel to (0 0 1) and is composed of four types of rigid boron-oxygen groups: two [B₅O_8.5]^2- pentaborate groups, [B₃O₅]^- triborate groups and BO₄ single tetrahedra. There are six symmetrically independent Rb atoms in the structure, of which four are located in voids within the borate layer and two are between the layers. High-temperature X-ray powder diffraction studies showed that the new compound forms above 620 °C as a result of the solid state reaction Rb₂B₄O₇ + α-RbB₃O₅ ↔ Rb₃B₇O₁₂. The compound melts at ∼ 700°C according to the peritectic reaction Rb₃B₇O₁₂ ↔ βRbB₃O₅ + liquid. Thermal expansion of the structure of Rb₃B₇O₁₂ is highly anisotropic (α₁₁ = 34, α₂₂ = 9, α₃₃ = 2 × 10^-6°C^-1). The thermal expansion is maximal in the direction perpendicular to the plane of the layer (α₁₁ = 34 × 10^-6°C^-1).