The distribution of hydrography and circulation in the eastern tropical Indian Ocean (ETIO) during April-May 2011 were analyzed using cruise observations, satellite observations, and historic hydrographic data. It was observed that warm water (>28°C) occupies the upper 50-m layer in the ETIO. Low-salinity surface water was observed at the mouth of the Bay of Bengal (BOB), which further extends to the Arabian Sea and off Sumatra via the Sri Lanka coast and the eastern bay mouth. Arabian Sea high-salinity water (ASHSW) is carried eastward along the equator to around 90°E by the equatorial undercurrent (EUC). It also runs south of Sri Lanka (north to 3°N) and in the western bay mouth (west to 87°E) but is much shallower than its counterpart at the equator. It is suggested to be the residual of the ASHSW, which intrudes into the BOB during the preceding southwest monsoon. Our results also show that, in the south of Sri Lanka, just below this subsurface high-salinity water, very-low-salinity water (about 34.8) occurs at depths of 100–200 m. Further analysis suggests that this low-salinity water comes from the BOB.