The [B12H12]2− anion shows an extensive substitutional chemistry based on its three‐dimensional aromaticity. The replacement of functional groups can be attained by electrophilically induced substitution caused by Brønsted or Lewis acidic electrophiles (e.g. Pt2+). Until now, it was impossible to structurally characterize a metal‐substituted [B12H12]2− cage. When an aqueous solution containing both Bi3+ cations and [B12H12]2− anions was heated, the charge‐neutral bismuth undecahydro‐closo‐dodecaborane BiB12H11 was obtained, representing a new class of metalated [B12H12]2− clusters. The title compound was characterized by single‐crystal X‐ray diffraction and NMR spectroscopic methods. Compared to the typical B−H bond, the short B−Bi single bond (230 pm) exhibits inverted polarity.