The crystal structure solution results (based on X-ray diffraction data) are presented for the last unknown structure in the series of alkaline metal triborates. The phase KBr₃O₅ was crystallized from the melt (≈800°C) in a metastable state using glass as precursor. The compound belongs to the new structural type, structure anion represents a single boron-oxygen skeleton formed from three independent rigid triborate rings [B₃O₅]^- consisting of two triangles BO₃ and one tetrahedron BO₄ characteristic for triborates. According to crystal-optic analysis, KBr₃O₅ crystals are biaxial, optically negative.