The tropical Indo–Pacific serves as a major source of energy and moisture and has great impacts on global climate system. Here we found the sea surface salinity exits a clear decadal seesaw pattern across the tropical Indo-Pacific with a cycle about 11 years based on Argo observation and several reanalysis and model products. These decadal salinity anomalies are firstly established in the western tropical Pacific and leads the salinity anomalies in the eastern tropical Indian Ocean by about two years and then propagated to the southeastern Indian Ocean. Budget analysis suggests two major contributors to this salinity seesaw, one is freshwater flux as a fast process, the other is meridional advection as a slow and phase-transition process. This decadal salinity seesaw is closely related to the tropical Pacific decadal oscillation and the transport of the Indonesian throughflow.