Prunella vulgaris L. is an important medicinal plant with a variety of pharmacological activities, but limited information is available about its response to potassium chloride (KCl) supplementation. P. vulgaris seedlings were cultured in media with four different KCl levels (0, 1.00, 6.00 and 40.00 mM). Characteristics relating to the growth, foliar potassium, water and chlorophyll content, photosynthesis, transpiration, nitrogen metabolism, bioactive constituent concentrations and yield were determined after three months. The appropriate KCl concentration was 6.00 mM to result in the highest values for dry weight, shoot height, spica and root weight, spica length and number in P. vulgaris. The optimum KCl concentration resulted in a maximum net photosynthetic rate (Pn) that could be associated with the highest chlorophyll content and fully open stomata conductance. A supply of surplus KCl resulted in a higher concentration of foliar potassium and negatively correlated with the biomass. Plants that were treated with the appropriate KCl level showed a greater capacity for nitrate assimilation. The Pn was significantly and positively correlated with nitrate reductase (NR) and glutamine synthetase (GS) activities and was positively correlated with leaf-soluble protein and free amino acid (FAA) contents. Both KCl starvation (0 mM) and high KCl (40.00 mM) led to water loss through a high transpiration rate and low water absorption, respectively, and resulted in increased concentrations of ursolic acid (UA), oleanolic acid (OA) and flavonoids, with the exception of rosmarinic acid (RA). Moreover, the optimum concentration of KCl significantly increased the yields of RA, UA, OA and flavonoids. Our findings suggested that significantly higher plant biomass; chlorophyll content; Pn; stronger nitrogen anabolism; lower RA, UA, OA and flavonoid accumulation; and greater RA, UA, OA and flavonoid yields in P. vulgaris could be expected in the presence of the appropriate KCl concentration (6.00 mM).