The recent success in the formation of chlorinated telluraboranes and the reactivities of pnictogenaboranes prompted us to re-examine the vacuum co-pyrolysis of B₂Cl₄ with Se₂Cl₂ at various molar ratios and temperatures in order to search for the generation of other polyhedral selenaboranes than closo-SeB₅Cl₅ (1a) and closo-SeB₁₁Cl₁₁ (1b), the latter being observed earlier. Interestingly, a new compound with the elemental composition SeB₉Cl₉ (2) was detected, this time by high- and low-resolution mass spectrometry. Further characterization by 1- and 2-D ¹¹B-NMR spectroscopy suggests that 2 should adopt a closed bicapped square-antiprismatic geometry with selenium at the apical position. Moreover, vacuum sublimation gave suitable crystals of 1b, which were subjected to single-crystal X-ray structure determination. Crystallographic data analysis confirmed that 1b, consistent with its 26 skeletal electron count, adopts a distorted icosahedral structure close to the symmetry of C_5v. Computations at the DFT-D3 level have revealed that 33% of the total computed binding motifs in the grown 1b crystals are due to the very strong chalcogen bonding. Moreover, SAPT decomposition has shown that the bonding motifs in the crystals are stabilized mainly by dispersion and electrostatic terms. Homodecoupling and high resolution ¹¹B NMR and ⁷⁷Se NMR experiments have resolved both coupling constants ¹J(¹¹B¹¹B) and ¹J(⁷⁷Se¹¹B) as well as the ⁷⁷Se chemical shift of 1a and 1b, which are in reasonable agreement with the corresponding computed values. The computed ¹¹B chemical shifts of 2 were determined by the well-established DFT/GIAO/NMR structural tool based on its B3LYP/6-311+G** internal coordinates. They agree well with the experimental values and provide a good representation of the molecular structure of 2 in solution. The extraordinary downfield ¹¹B NMR chemical shift of B(10) in 2 has been ascribed to the intensive paramagnetic contribution to the shielding tensor in this bicapped square-antiprismatic motif. Calculations of the synproportionation free energies of smaller (n – 1) closo-selenaboranes with larger-sized (n + 1) ones support the extraordinary stability of octahedral, bicapped square-antiprismatic and icosahedral closo motifs in the SeB_nCl_n family (n = 4–12).