Reaction between [BiBr₆]³⁻ and Br₂ in HBr in presence of trimethylammonium bromide results in self-assembly of a supramolecular halometalate-polybromide derivative (Me₃NH)₃{[Bi₂Br₉](Br₂)} belonging to a novel structural type. DFT calculations allowed estimation of Br⋯Br contact energies (1.3–4.4 kcal/mol).