The present study evaluated the physiological, quantitative, and qualitative responses of sugar beet (Beta vulgaris L. cv. Shokofa) to salinity stress and potassium sulfate application under greenhouse conditions. The experiment was conducted as a factorial arrangement based on a completely randomized design. Salinity stress markedly increased root Na⁺ concentration while significantly reducing K⁺ accumulation, resulting in ionic imbalance. These changes were accompanied by substantial declines in membrane stability index (MSI), relative water content (RWC), chlorophyll and soluble protein contents, root diameter and weight, leaf number, sugar content, recoverable sugar, sugar extraction coefficient, and leaf dry matter percentage. Application of 50 kg ha⁻¹ potassium sulfate alleviated some of the detrimental effects of salinity by reducing root Na⁺ concentration by 20.9% and increasing soluble protein content, root length, and sugar content by 15.44%, 3.49%, and 2.15%, respectively, compared with the control. Potassium supplementation also improved root ionic balance and contributed to the maintenance of leaf chlorophyll content under saline conditions. However, these improvements were insufficient to counteract salinity-induced reductions in membrane stability and plant water status, as evidenced by the non-significant interaction between salinity and potassium for MSI and RWC. Overall, the findings indicate that potassium application partially mitigates the adverse effects of salinity by enhancing ionic homeostasis and preserving photosynthetic capacity. Nevertheless, under severe salinity, membrane integrity and plant water relations are predominantly constrained by osmotic stress, limiting the effectiveness of potassium fertilization. Therefore, the application of an appropriate potassium dose, particularly 50 kg ha⁻¹ potassium sulfate, can be considered an effective management strategy for improving salinity tolerance and enhancing selected quantitative and qualitative traits of sugar beet.