Increased tree mortality due to extreme drought events in the Amazon has led to significant forest degradation and carbon stock reductions. Plants' ability to withstand water stress is primarily linked to the hydraulic traits of the species, such as water safety margins and phenotypic characteristics. However, some studies suggest that soil nutrients can play a modulating role in plants' responses to water stress. Among these, potassium (K) is a crucial cation for plants, being one of the most demanded elements. It is widely redistributed in plant tissues due to its high mobility and plays roles in energy production, translocation, and storage of assimilates, as well as maintaining water in plant tissues. Thus, this study aimed to evaluate the morphophysiological responses of two native Amazonian tree species (Enterolobium schomburgkii and Hymenaea courbaril) subjected to potassium fertilization and water restriction. We examined two native Amazonian species, one fast-growing (Enterolobium schomburgkii) and one slow-growing (Hymenaea courbaril), in a factorial design (2 x 2 x 4) involving two species, two water conditions (adequate irrigation and water deficit), and four potassium doses (0, 50, 100, and 200 mg/dm³), totaling eight treatments with 12 replicates, and 192 plants in total. We assessed height, diameter, leaf number, and height-to-diameter ratio, as well as gas exchange, functional traits, potassium concentration and accumulation, and potassium use efficiency. Growth in height and diameter was greater in treatments with higher potassium concentrations, and these plants were less affected by water stress. Overall, higher potassium concentrations reduced plant transpiration and enhanced water use efficiency. E. schomburgkii plants under higher potassium levels and water stress showed significant increases in water use efficiency. Potassium treatments and water deficit significantly affected leaf functional traits, with variations between the studied species. Leaf area was smaller in treatments with higher potassium doses for E. schomburgkii, with the opposite effect observed for H. courbaril. There was a reduction in specific leaf area (SLA) in treatments with higher potassium doses for E. schomburgkii, and an increase in SLA under water deficit for H. courbaril. Our results highlight the critical role of soil potassium concentration in influencing transpiration rates and water use efficiency, especially for E. schomburgkii, a fast-growing species. This suggests that variations in nutrient availability can influence tree physiology and their ability to withstand extreme droughts. Advancements in understanding these processes can aid in conservation actions and management strategies that enhance the resilience of tropical forests in the face of climate change.