A new barium borate halide, Ba₃B₁₀O₁₇Br₂, has been obtained. A detailed structural comparison with other barium borate halides suggested that Ba₃B₁₀O₁₇Br₂ is the first barium borate halide with a B–O layered structure. First-principles theoretical studies were conducted to aid understanding of the electronic structure and optical properties.