Changes in sea surface salinity (SSS) in the western Pacific warm pool (WPWP) provide information on the variability of the El Ni¿o Southern Oscillation (ENSO) because SSS reflects the balance between evaporation and precipitation. We reconstructed the long-term variability of SSS at the northwestern edge of the WPWP from oxygen isotope analysis of annual coral bands. The estimated SSS corresponded well with in situ observations, demonstrating that isotopic records from coral can be used as a paleo-salinometer. Estimated SSS between 1954 and 2000 peaked during the El Ni¿os of 1972--73, 1982--83, and 1997--98. These SSS peaks were caused by anomalously low precipitation accompanying the eastward shift in the WPWP.