<p>Potassium (K⁺) deficiency constitutes a major abiotic stress that negatively affects plant growth and productivity worldwide. Hydrogen sulfide (H₂S), a gaseous signaling molecule, plays an important role in regulating numerous developmental processes and stress responses throughout the plant life cycle. However, its role in mitigating the adverse effects of K⁺ deficiency remains poorly understood. In this study, we investigated the role of exogenous NaHS (an H₂S donor) in alleviating K⁺ deficiency-induced stress in <i>Brassica napus</i>. Plants were cultivated under two K⁺ conditions: sufficient K⁺ (3 mM, SK) or low K⁺ (20 µM, LK), with or without 200 µM sodium hydrosulfide (NaHS). Plant growth, K⁺ dynamics parameters, and enzymatic and non-enzymatic (polyphenols) antioxidants as well as HPLC analysis of leaf phenolic profile were determined. K⁺ deficiency reduced plant growth and K⁺ status while triggering oxidative stress. Exogenous application of NaHS significantly alleviated these effects by enhancing plant growth and improving K⁺ use efficiency (KUE). NaHS mitigating effect was also associated with modulation of enzymatic antioxidants [superoxide dismutase (SOD), catalase (CAT), and guaiacol peroxidase (GPX)], enhancement of non-enzymatic antioxidant capacity (as leaf total polyphenol content, DPPH radical scavenging activity, and individual phenolic compounds), and restored membrane stability, as evidenced by reduced electrolyte leakage. Overall, NaHS addition may be considered an effective strategy for promoting rapeseed growth under low K⁺ conditions by restoring plant biomass, improving KUE, and modulating antioxidative response.</p>