The title compound, s-fac-[Co(dien)2][B7O9(OH)6]·9H2O (1) (dien = HN(CH2CH2NH2)2), has been prepared as a crystalline solid in moderate yield (35%) from the reaction of B(OH)3 with [Co(dien)2][OH]3 in aqueous solution (10:1 ratio). The structure contains a novel polyborate anion [B7O9(OH)6]3 − which is structurally based on the known ‘ribbon’ isomer of [B7O9(OH)5]2 −, with an additional [OH]− group coordinated to a B atom in one of the outer boroxole rings. Compound 1 is formed by a self-assembly process in which the cation and anion mutually template themselves from equilibrium mixtures under reaction conditions. The [B7O9(OH)6]3 − anions are H-bonded to each other in layers with ‘cavities’ suitable for the [Co(dien]2]3 + complex. Three [B7O9(OH)6]3 − anions are in the secondary coordination sphere (via H-bonds) of each cation, with each anion H-bonded to three cations.