Single crystals of a new compound, Ba ₂Bi ₃B ₂₅O ₄₄, were grown by cooling a melt of non-stoichiometric composition. The crystal structure was solved by direct methods and refined to R=0.030 (wR=0.090). The compound is trigonal, R3̄ m, a=7.851(2), c=46.20(1) Å, V=2466.6(6) Å ³, Z=3. The structure is based upon a borate layered anion of the type not previously observed in inorganic compounds. The layer consists of three sublayers; it is ∼13 Å thick and oriented parallel to (001). Two of the sublayers are built from triborate groups, [B ₃O ₈] ^7-, whereas the central sublayer consists of triborate groups, [B ₃O ₆] ^3-. The interlayer space is occupied by the Bi ³ ions in octahedral coordination. The Ba ² cations are located in the cavities within borate layer. The single-phase polycrystalline sample of Ba ₂Bi ₃B ₂₅O ₄₄ prepared by solid-state reactions from a stoichiometric mixture has been investigated by high-temperature X-ray powder diffraction in air. Anisotropic character of the thermal expansion is not typical for layered structures: α _a=12, α _c=6, α _V=30×10 ^-6 ° ^-1 at 25 °; anisotropy increases on heating: α _a=12, α _c=0, α _V=24×10 ^-6 ° ^-1 at 700 °.