In this paper, the historical hydrographic database for the south Indian Ocean is used to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the sub-tropical front (STF), and especially (ii) the southern current band of the gyre. A current band of increased zonal speeds in the upper 1000 m is found just north of the STF in the west near South Africa and at the surface STF in the open Indian Ocean until the waters off the coast of Australia are reached. As neither any other investigation of this current nor a name for it are known, the flow has been called the South Indian Ocean Current (SIOC). This name is anologous to the same current band in the South Atlantic Ocean, the South Atlantic Current. The STF is located in the entire south Indian Ocean near 40°S. The associated current band of increased zonal speeds is the SIOC, which is found at or north of the STF. East of 100°E the SIOC separates from the STF and continues to the northeast. The zonal flow south of the STF is normally weak and serves to separate the South Indian Ocean and Circumpolar currents. New Africa the SIOC has a typical volume transport of 60 Sv (1 Sv ≡ 106 m33 s−1) in the upper 1000 m relative to deep potential density surfaces of σ4 = 45.87 kg m−3 (2800–3500 m) or σ2 = 36.94 kg m−3 (1500–2500 m). Near western Australia the SIOC is reduced to about 10 Sv as it turns to the northeast.