This study aimed to evaluate the effect of conventional potassium fertilization and Nano potassium (Nano-K) foliar application on the vegetative growth of alfalfa (Medicago sativa L.) under different water stress levels. The experiment was conducted using a factorial arrangement within a Randomized Complete Block Design (RCBD) with three replicates, including potassium source (conventional and nano) and irrigation levels (100%, 75%, 50%, and 25% of field capacity). The results revealed that Nano-K fertilizer application significantly improved all vegetative growth parameters compared to conventional potassium and control treatments. The concentration of 2 g L⁻¹ Nano-K recorded the highest values for plant height (47.08 cm), number of leaves (31.83 leaves plant⁻¹), fresh weight (3.48 g plant⁻¹), and dry weight (0.64 g plant⁻¹), while 3 g L⁻¹ showed superiority in leaf area (26.04 cm²). Conventional potassium also improved growth traits, particularly at 4 g kg⁻¹ soil, but remained less effective than Nano-K. Water stress significantly reduced most growth parameters, with the lowest values observed at 25% field capacity. However, Nano-K application alleviated the adverse effects of water stress, particularly under moderate and severe stress conditions. In conclusion, Nano-K fertilization, especially at 2 g L⁻¹, can be recommended as an effective strategy to enhance alfalfa growth and improve tolerance to water stress, offering a promising alternative to conventional potassium fertilizers.