Observational stable isotope data and model simulations show that the δ18O composition of precipitation in East Africa is closely related to the coupled ocean-atmosphere system over the Indian Ocean. During the rainy seasons δ18O is a very sensitive recorder of the Indian Ocean Zonal Mode (IOZM). The link between δ18O and the IOZM is established through adjustments in the large-scale overturning circulation over the Indian Ocean and associated changes in convective activity and precipitation over East Africa. The IOZM is recorded as significant departures in δ18O not only over East Africa but as a dipole mode with opposite sign to the east and west of the Indian Ocean.