In arid and semi-arid regions, salinity stress is considered one of the major limiting factors for sugar beet production. This study was conducted to investigate the interactive effects of salinity and potassium sulfate application on antioxidant enzyme activities and selected quantitative and qualitative traits of sugar beet (Shokoufa cultivar) under greenhouse conditions. The experiment was conducted as a factorial arrangement in a randomized complete block design with three replications. Treatments consisted of five levels of irrigation water salinity (0, 5, 10, 15, and 20 dS m⁻¹) and two levels of potassium sulfate (0 and 50 kg ha⁻¹). The results showed that increasing salinity significantly affected the activities of catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL). In addition, increasing salinity stress intensity resulted in greater accumulation of the compatible osmolytes proline and glycine betaine, indicating the activation of plant defense mechanisms and osmotic adjustment in response to salinity stress. Application of potassium sulfate at 50 kg ha⁻¹, compared with no potassium sulfate application, alleviated some of the adverse effects of salinity and improved the biochemical status and selected quantitative and qualitative traits of the plant. Overall, the results indicated that adequate potassium supply can enhance sugar beet tolerance to salinity stress by strengthening antioxidant defense mechanisms and osmotic adjustment. Therefore, appropriate potassium nutrition management can be considered an effective strategy for mitigating the adverse effects of salinity and maintaining sugar beet yield and quality under saline conditions.