<p><i>Eucalyptus</i> plantations are essential for economic and environmental sustainability, while providing benefits such as carbon sequestration and biodiversity conservation. Given Brazil’s dependence on potassium (K⁺) imports for eucalyptus cultivation, sodium (Na⁺) was explored as a cost-effective alternative to optimize nutrient dynamics and enhance drought resilience. A controlled greenhouse experiment was carried out employing different K/Na ratios to investigate the effects of partially substituting K⁺ with Na⁺ on the nutritional status, growth, leaf gas exchange and water-use efficiency (WUE) of two <i>Eucalyptus</i> species (<i>E. grandis</i> and <i>E. camaldulensis</i>) under varying water regimes. The combined supply of K and Na, particularly at a 70/30% K/Na ratio, significantly improved concentration, accumulation, and uptake efficiency of various elements and enhanced K-utilization efficiency, while also maximized growth, with root, stem, and leaf dry mass increasing by up to 50% under well-watered conditions and by 35% under water deficit conditions, relative to K-supplied plants. Additionally, the combined nutrient strategy enhanced WUE by optimizing stomatal conductance and reducing transpiration, particularly in <i>E. camaldulensis</i>, which demonstrated higher drought tolerance. These findings suggest that Na should partially replace K in eucalyptus fertilization, potentially reducing production costs while maintaining or improving growth and drought resilience. This study provides insights into sustainable nutrient and water management strategies that enhance productivity and resilience in forestry plantations under climate change scenarios.</p>