The Teesta River flows from India to Bangladesh and has been suffering from severe water diversion during the dry season by both countries since 2000. The Gajaldoba Barrage in India and the Dalia Barrage in Bangladesh have reduced the Teesta River’s dry-season flow from 6,000 to below 1,000 cusec, harming fisheries, navigation, ecosystems, and livelihoods, and causing siltation that increases monsoon flood risk. This study carried out a quantitative analysis of the hydrology (water balance) including water inflow, water diversion, and groundwater inflow to the river during the dry season (January to April). Flow/discharge data from 2005 to 2010 were available at three locations; Gajaldoba (India), Dalia (Bangladesh), and Kaunia (Bangladesh). The information of groundwater inflow into the river was not available directly, were calculated from the water balance equation. The study found that the average amount of water diverted during the dry season by the Gajaldoba barrage is 68.9% of flow available at that point and the water diverted by the Dalia barrage is 56.5% of the flow available at that point. On the other hand, 67.8% of the flow at Dalia is groundwater inflow between Gajaldoba and Dalia (100 km). Similarly, 73.3% of flow at Kaunia is groundwater inflow between Dalia and Kaunia (60 km). The study discovered that the major inflow to the river during the dry season is the groundwater inflow.