The space-borne measurements of the SMOS mission reveal for the first time the complete features of the sea surface salinity (SSS) signature at the full scale of the Pacific basin. The SSS field in the equatorial cold tongue is typically found to be larger than 35.1 within a narrow 2° band of latitude that is positioned slightly south of the equator and that stretches across the eastern Pacific basin up to the Galapagos Islands. On the northern edge of the eastern equatorial Pacific this signature results in a very strong horizontal gradient (larger than 2 units over 100 km) with the fresh waters of the Panama warm pool. By considering a water density criterion, it can be shown that the cold tongue is characterized by a strong seasonal cycle with a 3°C amplitude in SST where the warm season of February-March contrasts with the cold period extending from September to November. If the present ocean reanalyzes are able to capture these features, then the assimilation of the SMOS data becomes a worthwhile objective in order to depict more accurately the salinity signature of the cold tongue of the tropical Pacific.