Soil salinity is one of the most significant environmental problems that negatively affect crop growth and productivity due to the high concentration of sodium (Na+) ions in agricultural lands. Amaranth (Amaranthus caudatus L.) has been proposed as a robust alternative to traditional cereal crops in areas likely to be affected by increased salinity in the future. This work investigates the physiological and biochemical responses of three genotypes of amaranth (‘Red’, ‘Green’ and ‘Pony’ genotypes) to different gradual levels of salinity (100 and 200 mM). It was shown that a treatment with 100 mM NaCl improved significantly leaf and root dry biomass with maintenance of water contentparticularly ‘Green’ present the best dry biomass. The net CO2 assimilation rate (PN), stomatal conductivity (gs), transpiration rate (E) and water use efficiency (WUE) decreased significantly with the intensity of saline stress. By the same, intercellular concentration of CO2 decreased in ‘Red’ and ‘Pony’ genotypes while it increased in ‘Green’ genotype. Therefore, salt tolerance in amaranth is strongly linked to ion homeostasis with an ability to accumulate Na+ in the stems to protect the leaves and to keep a high K+/Na+ ratio in the leaves. A significant increase in antioxidant enzyme activity; catalase (CAT), ascorbate peroxidase (APX) and guaiacol peroxidase (GPOX) in leaves and roots and an accumulation of proline were noticed. Amaranth may then be considered as a promising crop in arid and semi-arid regions affected by salinity.