Na + can substitute K + under K + deficiency conditions, which is denominated as beneficial Na +. Despite its relevance at the physiological level and its interest in agriculture, the mechanisms allowing its uptake and use by plants remain unclear. The accumulation of beneficial Na + has been associated with K + deficiency, but the deficiency of other mineral nutrients could also affect this process. In this study, we observed that the combined deficiency of K + and P (–K –P) resulted in a 2-fold increase in the root Na + content in tomato plants with respect to the single K + deficiency (–K). Such a response was observed also in quinoa and barley but not in the other five plant species, evidencing natural variability for this trait. Na + accumulation in –K –P plants was not related with Pi uptake as deduced from membrane depolarization and plant growth experiments. Finally, we show that the Na + channel HKT1;1, but not HKT1;2, contributes to beneficial Na + uptake in tomato plants, in particular, in the –K –P condition where a 40-45% reduction in root Na + content is observed in HKT1;1-silenced lines. Therefore, the information acquired on HKT1;1 is crucial for boosting beneficial Na+ uptake in plants.