THE EXPERIMENT was conducted to investigate the effects of application of rice straw waste-derived biochar on the growth and some metabolic activities of salt-stressed cowpea (Vigna unguiculata L.). Moreover, biochar as a soil amendment was evaluated with the additional rate of chemical fertilizers (NPK) under salinity stress in agro-ecosystem. The results indicate that salinity (200mM NaCl) caused a significant reduction in all measured growth parameters of cowpea seedlings compared with control. Salinity stress caused reduction in germination percentage by 54% and 14% and 17% in the lengths of shoot and root, respectively as well as leaf area (22% reduction), compared with the control. Furthermore, the activities of antioxidant enzymes (peroxidase, catalase and superoxide dismutase), malondialdehyde (MDA), ascorbic acid content and osmolytes (total soluble proteins, carbohydrates and amino acids) were increased under salinity stress. The results showed that application of rice straw-derived biochar increased the germination percentage and all the studied growth criteria under salinity stress. In addition, rice straw-derived biochar increased the photosynthetic activity and photosynthetic pigments (Chl. a, Chl. b and carotenoids) and decreased the biosynthesis of MDA and ascorbic acid. The results demonstrate that the highest values of the measured growth parameters and physiological processes of cowpea seedlings were recorded at 75% NPK combined with biochar treatment. In conclusion, application of rice straw-derived biochar reduced the additional rate of chemical fertilizer by 25%.