AbstractThe new hydroxyl barium borate BaB8O11(OH)4 was synthesized by using a low‐temperature molten salt technique with boric acid as flux at 458 K. Its crystal structure was determined from single‐crystal X‐ray data. The compound crystallizes in the monoclinic space group P121/n1 (No.14) with unit cell of a = 790.80(16) pm, b = 1393.9(3) pm, c = 1004.7(2) pm, β = 90.00(3)°, V = 1107.6(4) × 106 pm3, Z = 2. It is isostructural to PbB8O11(OH)4, but obviously different from AB8O11(OH)4 (A = Ca, Sr) in cell. The crystal structure of BaB8O11(OH)4 is built from corrugated borate layers,{[B8O11(OH)4]2–}n parallel to (010), which are stacked along the b axis and are linked by barium atoms. The borate layer consists of [B6O9(OH)] clusters as fundamental building blocks (FBBs), which are connected with surrounding four FBBs by sharing common oxygen vertices to form a sheet, and the sheet is further decorated with the flanking [B2O2(OH)3] dimers. FT‐IR and Raman spectroscopy, DTA‐TG, and the identification of an intermediate phase are presented.