Purpose <p>More research is essential to determine the effects of potassium (K) fertilization on yield and quality of Persian clover (<i>Trifolium resupinatum</i>) forage in drought stress conditions as objective of present research.</p> Methods <p>The two-year experiment was conducted as a split plot including drought stress (main plots, 50, 65 and 80% of water depletion) and K fertilization (K₂SO₄) (subplots, control, 100, 125, 150, 175 and 200&#xa0;kg ha^− 1) (Total of 18 treatments). Clover forage fresh (Yw) and dry (Yd) biomass, harvested at three different times, and physiological properties (protein and nutrient contents and water use efficiency (WUE)) were determined.</p> Results <p>Drought stress, year, harvest and year in harvest significantly affected forage yield, nutrient uptake and WUE. Control drought resulted in the highest Yw (43.26 tons ha^− 1) and Yd (13.25 tons ha^− 1) and severe stress resulted in the highest WUE (1.91&#xa0;kg m^− 3). Potassium fertilization significantly affected Yd and WUE, but not Yw. Potassium fertilization at 175&#xa0;kg ha^− 1 resulted in the highest Yw (39.87 tons ha^− 1), Yd (12.47 tons ha^− 1) and WUE (1.92&#xa0;kg m^− 3, 12% higher than zero potassium). Year and harvest significantly affected protein content but not drought and K fertilization. Year and drought significantly affected plant nutrient uptake and K fertilization was just significant on K uptake.</p> Conclusions <p>The use of potassium fertilization, especially at 175&#xa0;kg ha^− 1, is recommendable for improving clover yield and quality in drought stress conditions as it enhances plant growth by improving plant physiological properties including nutrient uptake and WUE.</p>