Increasing salinity of water resources, especially in arid and semi-arid regions of the world and Iran, is a major challenge in agricultural production that limits plant growth and performance. However, the use of compounds such as sodium nitroprusside, a nitric oxide donor, can enhance plant tolerance to salt stress. In this study, the effects of different salinity levels (0, 3, 6, and 9 dS/m) and the application of 100 µM sodium nitroprusside on morphological, physiological, and biochemical traits of sweet william (Dianthus barbatus) were investigated. The results showed that increasing salinity significantly reduced plant growth and performance, especially at higher levels (6 and 9 dS/m). Application of sodium nitroprusside effectively mitigated the negative effects of salinity at low to moderate salinity levels (0, 3, and 6 dS/m), but its protective effects decreased at the highest salinity level (9 dS/m). Additionally, sodium nitroprusside increased photosynthetic pigments, reduced ion leakage, maintained relative leaf water content and leaf potassium, and enhanced proline accumulation, soluble protein content, and antioxidant enzyme activities including superoxide dismutase and catalase. These results indicate that sodium nitroprusside can partially alleviate the adverse effects of salinity in sweet william.