Single crystals of a new compound, Ba2Bi3B25O44, 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, R3¯m, a=7.851(2), c=46.20(1) Å, V=2466.6(6) Å3, 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, [B3O8]7−, whereas the central sublayer consists of triborate groups, [B3O6]3−. The interlayer space is occupied by the Bi3+ ions in octahedral coordination. The Ba2+ cations are located in the cavities within borate layer.The single-phase polycrystalline sample of Ba2Bi3B25O44 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: αa=12, αc=6, αV=30×10−6 °С−1 at 25 °С; anisotropy increases on heating: αa=12, αc=0, αV=24×10−6 °С−1 at 700 °С.