The genes encoding the HAK5 transporters are induced by K+ starvation and repressed by long-term exposure to high Na+. This occurs in parallel with the hyperpolarization and depolarization of root cell membrane potential. In the present study it is shown in tomato plants that the presence of high Ca2+ during the K+ starvation period that leads to LeHAK5 induction, counteracts the repression exerted by high Na+. High Ca2+ reduces the Na+-induced plasma membrane depolarization of root cells, resorting one of the putative first steps in the low-K+ signal cascade. This allows proper LeHAK5 expression and functional high-affinity K+ uptake at the roots. Thus, the maintenance of HAK5-mediated K+ nutrition under salinity by high Ca+ can be regarded as a specific beneficial effect of Ca+ contributing to salt tolerance in plants.