The existence of a seasonally varying undercurrent along 8° N off the east coast of Sri Lanka is inferred from Conductivity-Temperature-Depth profiles, Argo floats, glider measurements, and Ocean General Circulation Model outputs. Together, they reveal an undercurrent below 200 m that is approximately 140 km wide and can reach a maximum speed of 45 cm s<sup>−1</sup> that hitherto has not been observed. The undercurrent, flowing in the opposite direction to the surface current, is most pronounced during boreal spring and summer and switches direction between these two seasons. The undercurrent transports relatively fresh water southward during spring, while it advects more saline water northward along the east coast of Sri Lanka during summer. This suggests a pathway, independent of the surface circulation, whereby freshwater is removed and saline water is injected into the Bay of Bengal.