<p>Potassium supplements should be a strategy to solve the water shortage problems in medicinal mondo grass production. The experiment evaluated the impact of potassium application on the growth, yield, and secondary metabolites of mondo grass under rainfed conditions. The split-plot design with three replications was used with the main factor of five potassium levels (0, 30, 60, 90, and 120&#xa0;kg K₂O ha⁻¹&#xa0;year⁻¹) and the sub-factor of two mondo grass genotypes (Mach Mon 2 and Mach Mon 6). Under rainfed conditions, increasing potassium supplement increased canopy size, the number of leaves and tillers, root growth, leaf osmotic pressure and chlorophyll fluorescence, total biomass, yield components, uptake of macronutrients, contents of polysaccharide, saponin, flavonoid, and decreased leaf water deficit and ion leakage. A potassium amount of 90&#xa0;kg K₂O ha⁻¹&#xa0;year⁻¹ seemed the optimum dose for productivity and medicinal quality of mondo grass under rainfed conditions.</p>