Boroxol (B 3 O 3 ) rings and relevant hexagonal B 3 S 3 structural blocks are ubiquitous in boron oxide/sulfide glasses, crystals, and high-temperature liquids. However, the isolation of an ultimate heterocyclic B 3 O 3 or B 3 S 3 cluster in the free-standing form, with as few as six atoms, has been unsuccessful so far. We report on computational design of the simplest case of such a system: the highly symmetric D 3h B 3 S 3It is the well-defined global minimum on the potential energy surface, following global searches and electronic structure calculations at the B3LYP and single-point CCSD(T) levels. Chemical bonding analysis reveals an ideal system with skeleton Lewis B S σ single bonds and unique double 6π/2σ aromaticity, which underlies its stability. The cluster turns out to be an inorganic analog of the 3,5-dehydrophenyl cation, a typical double π/σ aromatic system. It offers an example for chemical analogy between boron-based heterocyclic clusters and aromatic hydrocarbons. Double π/σ aromaticity is also a new concept in heterocyclic boron clusters. Previous systems such as borazine, boroxine, and boronyl boroxine only deal with π aromaticity as in benzene. K E Y W O R D S boron sulfide clusters, boron-based heterocyclic clusters, boroxol ring, chemical bonding, double π/σ aromaticity 1 | INTRODUCTION The intrinsic electron deficiency of elemental boron leads to its rich chemistry. Boron-based nanoclusters show unique structural and electronicproperties, as well as novel chemical bonding governed by π/σ aromaticity, antiaromaticity, and conflicting aromaticity. 1-17 Anion boron clusters were found to assume planar or quasi-planar (2D) geometries in a wide range of sizes up to 40 atoms, 9,15 which feature triangular close-packed 2D structures that are decorated by defect holes. Borospherenes 15,18-20 were observed as "cubic cages" with interwoven boron double chains, whose surfaces have hexagonal vs heptagonal holes. Boron double chains were also observed as structural blocks in tubular boron clusters 8 and extended 2D boron materials such as borophenes. 21,22 Defect holes (including hexagonal holes) in low-dimensional boron systems help compensate for their electron deficiency. For the same reason, monocyclic rings or single chains are nonexistent in bare B clusters, in contrast to carbon. 23 Chemical analogy has been established systematically in recent years between boron clusters and hydrocarbons. 9,24,25 Intuitively, oxidation or sulfuration of boron clusters results in boron oxide or sulfide clusters, 26 which should be even more electron deficient because such processes withdraw electrons from boron. Nonetheless, binary B O/B S clusters can form heterocyclic structures and make use