To control net sodium (Na + ) uptake, Arabidopsis plants utilize the plasma membrane (PM) Na + /H + antiporter SOS1 to achieve Na + efflux at the root and Na + loading into the xylem, and the channel-like HKT1;1 protein that mediates the reverse flux of Na + unloading off the xylem. Together, these opposing transport systems govern the partition of Na + within the plant yet they must be finely co-regulated to prevent a futile cycle of xylem loading and unloading. Here, we show that the Arabidopsis SOS3 protein acts as the molecular switch governing these Na + fluxes by favoring the recruitment of SOS1 to the PM and its subsequent activation by the SOS2/SOS3 kinase complex under salt stress, while commanding HKT1;1 protein degradation upon acute sodic stress. SOS3 achieves this role by direct and SOS2-independent binding to previously unrecognized functional domains of SOS1 and HKT1;1. These results indicate that roots first retain moderate amounts of salts to facilitate osmoregulation, yet when sodicity exceeds a set point, SOS3-dependent HKT1;1 degradation switches the balance toward Na + export out of the root. Thus, SOS3 functionally links and co-regulates the two major Na + transport systems operating in vascular plants controlling plant tolerance to salinity.