Abstract: Seasonal variation of the surface-layer circulation in the eastern tropical Indian Ocean (ETIO) is analyzed using nearly 20 years of surface absolute dynamic topography (ADT) data derived from satellite observations, ocean surface current analysis-realtime (OSCAR) data and the trajectories of Argos surface drifters. The results show that the seasonal variations of the surface circulation and the monsoons in the ETIO are almost coinstantaneous. The seasonal variations are more significant north of the equator. With this seasonal cycle of large-scale circulation, the circulation at the mouth of the Bay of Bengal changes accordingly. During May-September, the eastern mouth of the bay is dominated by the southward meridional current, which extends further southward off Sumatra, while it reverses to flow northward during the rest months. The meridional current in the western bay mouth is generally opposite to that in the east. The Argos drifting buoy trajectories further reveal that the water exchange pathways in and out the bay change seasonally. During the summer monsoon, the drifting buoys from the southern India Ocean and the Arabian Sea are mainly drifted into the bay from the west side of the bay mouth, while those from the bay are drifted out from the east side. The pathway reverses during the winter monsoon. This study also shows that the maximum variance of the zonal currents is caused by the monsoon currents including the Wyrtki Jet, while the maximum variance of the meridional currents is due to the East Indian Coastal Current (EICC) and Lakshadweep High (Lakshadweep Low).