The discovery of cycloosmathioborane compounds is reported. These species, which are prepared by the simultaneous dehydrogenation of a trihydride hydrogensulfide osmium(IV) complex and a BH3NHR2 amine-borane, bear an Os-S-B three-membered ring, being a manifestation of the 4n + 2 Huckel aromaticity in which n = 0 and where the two pi electrons of the ring are provided by the S atom.