Black crystals of trithallium(I) enneaiododibismuthate(III) Tl ₃Bi₂I₉ were obtained by slow cooling of a melt with composition Tl:Bi:I = 3:2:9 from 800 K to ambient temperature. The compound is diamagnetic and decomposes peritectically at 619(2) K. X-ray diffraction of single-crystals revealed that Tl₃Bi₂I₉ crystallizes in the monoclinic space group P2₁ln with lattice parameters a = 1438.9(1), b = 771.54(7), c = 2091.0(1)pm and β = 93.309(6)°, (Z = 4). The crystal structure can be described as a defect variant of the perovskite type (ABX₃), where according to the formula Tl(Bi_2/3□J_1/3)I₃ the Bi atoms occupy only two thirds of the octahedral sites B. The vacancies are located in every third plane resulting in double layers of corner sharing [BiI₆] polyhedra. By distortion and rotation of the [BiI₆] polyhedra the coordination of the Tl atoms (cuboctahedral sites in the perovskite type, c.n. = 12) is reduced to bicapped trigonal prisms (c.n. = 8). The group-subgroup relation between the Tl₃Bi₂I₉ structure and the perovskite type results in a subgroup index of 144.