Algorithms are developed for the estimation of sea surface salinity (SSS) in the tropical Indian Ocean from the space-borne satellite measurements of outgoing longwave radiation (OLR). The algorithms are based on the interrelationships between OLR, the effective oceanic layer (EOL), climatological SSS (World Ocean Atlas 1998 (WOA98)), and freshwater flux (P-E). Coupling between the atmospheric convection (OLR) and the geopotential thickness of the oceanic near-surface stratified layer (EOL) forms the main relationship for the estimation of SSS, followed by the relationships between EOL and WOA98 SSS and between OLR and P-E. The SSS has been estimated using 16 years (1979--1994) of OLR data and the algorithms for the tropical Indian Ocean. The estimated SSS at 2.5¿ ¿ 2.5¿ grid on monthly scale is nearer to the WOA98 SSS with lower differences within ¿0.5--0.8 away from the coastal region. The estimated SSS also agrees reasonably with the observed SSS along the trans-Indian zonal sections occupied during World Ocean Circulation Experiment (WOCE) and also other sections. The SSS differences bring out clearly the impact of the strong 1997--1998 El Ni¿o on the rainfall in the southeastern Indian Ocean and the eastern equatorial region. On the basis of the results, the seasonal and interannual variability of SSS in the tropical Indian Ocean are discussed. Through a physical model, phase lags between P-E and SSS and between OLR and P-E are estimated and their incorporation into the algorithms is suggested for improvement in the estimation of SSS. This SSS information is useful to the studies on coupled models, El Ni¿o-Southern Oscillation forecast models, and Ocean Global Circulation Models or regional scale circulation models.