Abstract: Projections of the equatorial Pacific zonal sea surface temperature (SST) gradient and Walker circulation exhibit substantial intermodel uncertainty. Through analyses of a suite of 54 CMIP5 and CMIP6 models, we show that this uncertainty is closely linked to Southern Ocean (SO) sea surface salinity (SSS). Models with a saltier SO surface waters feature a denser and less stratified upper ocean, allowing greenhouse-warming-induced heat uptake to penetrate more deeply and thereby suppressing local surface warming. Through previously identified SO–southeastern Pacific (SEP) teleconnection, this reduced SO warming mitigates the weakening of both the equatorial Pacific zonal SST gradient and Walker Circulation. A CESM2-based salinity nudging experiment further supports this proposed physical mechanism. Combining this intermodel relationship with observed SO SSS, we develop an emergent constraint showing that the widespread fresh bias in simulated SO salinity contributes to overestimates of the future weakening of equatorial Pacific SST gradients and atmospheric circulation. These results highlight SO salinity as a critical source of uncertainty in tropical Pacific projections and suggest that reducing SO salinity biases could improve simulations of future Pacific air–sea interactions.