N-type bismuth telluride sulfide has received attention as a potential thermoelectric material due to its large thermopower and ability to form solid solutions with bismuth antimony sulfide. Here, we fabricated an n-type tetradymite BiSbSTe2 by solid-state reaction. The crystal structure was found to be rhombohedral, and the thermoelectric properties were measured for the temperature range 50–390 K. A high Seebeck coefficient of −190 μVK−1 was achieved at 385 K. The electronic structure of BiSbSTe2 was investigated using first-principles calculations and compared with its parent counterpart (Bi2STe2). The results have shown that BiSbSTe2 is a narrow band-gap semiconductor. For the first time, we have demonstrated that the vacancies in substitutional atomic sites, which can be altered through different fabrication techniques, can determine the charge carrier nature of tetradymite thermoelectrics. As a proof of concept, we have shown that the vacancies in antimony sites result in a p-type compound while a sulfur deficiency produces an n-type phase with lower electrical conductivity.