Abstract <p>Increasing extreme temperatures associated with global climate change are the primary cause of yield losses in spinach (<i>S. oleracea</i>), a cool season vegetable. In this study, the effects of exogenous sodium nitroprusside (SNP) applications on the morphological development and antioxidant defense of spinach seedlings exposed to 40°C heat stress were investigated. The findings revealed that heat stress suppressed LN, plant height, and biomass accumulation, accompanied by a decrease in chlorophyll content and an increase in lipid peroxidation (MDA) and H₂O₂ accumulation. Although CAT activity increased in the heat control group as a natural resistance mechanism, this response was found to be insufficient in preventing cellular damage. However, 25 µM SNP application increased CAT activity by 93.6% compared to the heat control group and reduced oxidative damage parameters below control levels. Correlation analyses confirmed significant relationships between increased CAT activity and decreased MDA and H₂O₂ levels, suggesting that SNP limits oxidative damage by activating defense mechanisms. Furthermore, the positive correlation between increased leaf number, improved growth parameters, and chlorophyll stability indicates that cellular level recovery is potentially reflected in plant performance. In conclusion, the application of 25 µM SNP is determined to enhance physiological performance and support heat tolerance in spinach seedlings under heat stress.</p>