New anhydrous lead borate Pb₂[B₅O₉]Br (sp. gr. Pnn2) was synthesized by the hydrothermal method. The second harmonic generation from polycrystalline samples of Pb₂[B₅O₉]Br is characterized by a higher signal than that observed in powdered LiB₃O₅. The crystal structure of the new hilgardite-group compound was refined by the Rietveld method. Analysis of the known orthorhombic polar varieties of hilgardites, including the new compound, showed that their boron-oxygen frameworks are occupied by the Pb²⁺, Ca²⁺, Eu²⁺, and Ba²⁺ cations; the Cl⁻, Br⁻, and OH⁻ anions; and water molecules in combinations determined by their sizes.